Fast de-excitation brushless exciter
Fast de-excitation brushless exciter
(OP)
I searched in the net material for fast de-excitation brushless exciter, but is there anyone who knows more about this subject and if he saw somewhere applied this operating mode.
Thank you.
Thank you.





RE: Fast de-excitation brushless exciter
I have spec'ed, installed and serviced a lot of generators in the 15 KVA to 1500 KVA range. The natural characteristics of the AVR were fast enough for load loss. Never had a problem. Standby sets would sometimes trip on over-speed when the load was abruptly dropped when switching back to the mains after an outage. That was a governor and setting issue. With 3% droop and no load the target frequency is 61.8 Hz (51.5 Hz). A little fast and the load dump would cause over-speed trips. A little slow and the UFRO would act prematurely.
A brushless exciter generates AC. This is rectified by the rotating diodes to supply the field. When the AVR cuts the output to the brushless exciter the output of the brushless exciter will drop almost instantly. The field and voltage will decay as the magnetic field collapses. The time to decay to zero is generally taken as 5 time constants, one time constant, in seconds being the product of the induction of the field coil in Henries and the resistance of the discharge path in Ohms. If that's not fast enough for you, you may have to come up with some type of rotating field forcing circuit to reverse bias the field current and force the current down.
In one time constant the variable will reach about 63% of the final value.
Bill
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"Why not the best?"
Jimmy Carter
RE: Fast de-excitation brushless exciter
RE: Fast de-excitation brushless exciter
If you look at the time constants in datasheets for machines in the size range that Waross mentioned, they're usually quite short, thus its not really a requirement.
I attended a Basler presentation last year where they discussed the aspects of 'reverse excitation', a feature only found in the larger specification AVRs. I admit I don't recall if it was specific to do with brushless excitation, as they also did a large presentation on static excitation (i.e. removing the exciter rotor / stator and connecting direct to the main field winding, normally with slip rings).
I would think it'd only come in for larger machines (probably about the size that ScottyUK deals in), certainly in the size range I deal in its not a requirement (which is about the same scale as Waross).
Have a look at the Basler website and see if they have any tech notes or application notes for it.
RE: Fast de-excitation brushless exciter
For waross..please give me more details about what is "you may have to come up with some type of rotating field forcing circuit to reverse bias the field current and force the current down."
RE: Fast de-excitation brushless exciter
Most generators will have quite a bit of remnant flux, so the generator will continue to feed the fault as it spins down regardless of excitation time constants.
RE: Fast de-excitation brushless exciter
As for fault response, I suspect mgtrp is right.
RE: Fast de-excitation brushless exciter
The typical exciter is a 3-phase pilot generator winding with 6 diodes attached feeding into the main field winding. You control the main field winding by controlling the field on the pilot generator. Overall, they rather simple setups.
To force the field off, you'd have to switch the rectifier to use 12 SCR's and a controller to for SCR's. Instead of dropping the pilot field to remove the excitation you'd have to be able to signal this controller to force the current to zero instead. In other words, you'd basically need a 4-quadrant DC drive rotating on the rotor shaft which you can control externally. It seems very unreasonable to expect to have that level of electronics rotating on the shaft.
RE: Fast de-excitation brushless exciter
RE: Fast de-excitation brushless exciter
To extinguish the arc, I would consider isolating the stator and interrupting the ground path. There may still be enough capacitive current to maintain an arc, but hopefully a less severe arc.
Bill
--------------------
"Why not the best?"
Jimmy Carter