Excitation system

[Shamas javeed]

Good evening

Dear all I have an issue in excitation system

The problem is when ever the unit gets shutdown over external fault The AVR card get burned (Flash out) but on normal shutdown the card works properly.
What may be the possible resions for this?

Protection on both ends of FCB is also installed Field discharge resistor on generator side and VDR on excitor side,
Regards

 

[RRaghunath]

When there is external fault that causes voltage dip at generator terminals, the excitation systems gets in to 'boost' mode, increasing the excitation current temporarily in order to feed more VARs and maintain the terminal voltage.
This is not the case when it is normal shutdown.
Apparently, you have problem in boost mode operation of AVR. Over to you for further investigation.

 

[Shamas javeed]

Thanks RRaghunath sir

Sir how to investigate the boost mode operation of AVR.?
The deexcitation command is auto generated with field breaker with nc contact.

 

[waross]

On a fault there is the possibility of pole slip.
Your rotating field suppression may not be robust enough to handle the transients developed by a pole slip.
Once the field is disconnected from the AVR the AVR may not be in danger but the damage may have already been done.
what size of generator?
What prime mover?
Age of the machine?
There is often a CT feeding back to the AVR to provide quadrature compensation for parallel operation.
One method of field boosting uses the rectified output of a different CT to provide field boost under excessive current conditions.


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

 

[Shamas javeed]

Waross sir

The generator is 850kw
Two generators sync total capacity is 1.7 mw (same Fault on both generators)
85 vdc field voltage
166 A field current
Hydel unit
About 10 years old machine hardly one year working life

there are only two cts in circuit one over star point and second on phases and two AVR cards working in parallel

The faults so far I have traced are
Surge arrestor at 11kv line not installed

Star point of generator not grounded

Eirthing of excitor Avr card is not proper

May be the VDR is not working properly

Further your king help is required
Regards

 

[Shamas javeed]

And sir how to know weather its pole slip??