How a crow bar works on excitation system

[smahsan]

Dear Sir I m not much aware of genarator and excitation system bcz i m related from boiler and turbine auxiliary plant operations but some time we face these faults.
Can u any one define me plz.
Tahanks and regards
S M Ahsan

 

[rbulsara]

be more clearer..

What type of faults?

And at leat to me, define crowbar, is that a black start? or a crow bar circuit that intentionally grounds a phase upon sensing of arcing to creat enough current to operate a fast fuse or something like that.

I must clarify, not an expert in utiltiy grade power plants or crow-bar circuits. Just want to understand what are you asking..others may help..

 

[RRaghunath]

Crow bar mechanism is associated with the excitation circuit breaker.

I am not sure why the name "Crow bar" but the function as I amd aware is,

When ever excitation to the generator is to be interrupted, the breaker opens and earths the excitaion winding through a discharge resistor.

 

[Skogsgurra]

You should be aware that the word "crowbar" or "crow-bar" is being used in many fields of technology. The more common is to avoid overvoltage in 5 V and similar circuits and that is what most people probably think of. That circuit is an SCR which triggers when voltage gets too high.

There are also crowbars in power distribution centers. The purpose is the same, but the voltage levels are kV and the technologies used are more varied. And then again is probably a myriad of other crowbars that very few of us ever got in touch with.

The way you describe "your" crowbar indicates that it is a discharge resistor that gets switched into the circuit to avoid inductive kick-back when the DC breaker opens. It is a problematic solution since there must be a very exact timing so that the resistor is switched into the circuit before any overvoltage develops. A tricky task, indeed, if you are using a breaker and auxiliary contacts. A permanently connected varistor (MOV) is usually used for this task nowadays. Or a diode/resistor combination if the excitation is unipolar, which it is as a rule.

You do not describe the case very well and you do not say what your failure mode or problem is. Please understand that you need to give all pertinent details if you want an answer that can help you out.

 

[RRaghunath]

Thanks Skogsgurra. I agree with your explanation.

Even in case of generators the purpose is to limit the overvoltages due to sudden interruption of excitation current when excitation circuit breaker is opened (there may be a surge arrestor as well in the circuit as you mentioned - I am not very sure).

Any idea why is it called "crow bar" circuit!

 

[Skogsgurra]

I always thought that the analogy with placing a piece of heavy metal across the poles to short the voltage out is the explanation.

A crowbar is a piece of iron that is used to break up boxes (and doors if you are a burglar)but it can also be used to short-circuit almost anything.

 

[smahsan]

Dear skogsgurra
I m now well aware as u defined or explained the function of crow bar and this is being used in generators excitation system to face over voltage or surge.I m not perticularly well know about electrical systems.U defined as i wished and its clear to me we had been facing same problem that the over voltage being passed via this crow bar system whichi s based on thyristors which dicipates the over current and avoid over volatge.Thanks for your good reply.
S M Ahsan