Hydro Generator field contactor & Discharge Resistor still necessary?

[CurrentKills]

I'm rehabilitating a 2MW hydro with rotating DC generator (compound wound). Replacing existing AVR with Basler DECS 250N which will energize the exciter shunt field. Given the DECS has a crowbar circuit and six SCR full wave output stage, Basler application engineer has told me that the generator main field contactor and discharge resistor should not be necessary.

Has anyone done an AVR replacement, maintaining the existing rotating exciter, and omitted the field contactor & discharge resistor? Any problems with over-voltage when the unit trips? Anyone measure the voltage decay on a test trip?

There have been a couple other threads on this topic, but they lacked closure...
-I'd also take recommendations for replacement field contactors (w/discharge pole). All I've found is ABB's "R Contactor" product line.

DC generator is rated 50kW, 125 VDC, 400A

Thanks all.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1448465333/tips/Exciter_dwg1_utqkh3.pdf[/url]

 

[catserveng]

I would go with what the Basler engineer told you. The DECS 250N is a negative forcing AVR, and as with all the DECS family of AVR's, a field contactor or breaker is NOT recommended as opening the field under load can cause a failure of the regulator.

How well it responds to trips and voltage decay is pretty much in the hands of whoever programs it. It has a large programming menu and most of the problems I see with these regulators is errors in setup and programming.

I'm assuming since you went with an "N" type regulator you have an inverting style excitation system? Getting that setup right can be a real pain, but when properly done the response and power system stability functions can work very well.

If you haven't done a retrofit with a newer DECS before, and don't have a set of "sample" settings, you may want to find someone who has done it to help you thru the first one, they can be quite a challenge.

Hope that helps, MikeL.

 

[CurrentKills]

Thanks Mike.
I'm not familiar with the "inverting style excitation system" you mention. Also not sure of the motivations behind the 250N selection, but another key difference is 20A output vs. 15A in the 250 (non-N), which I'm not sure is necessary to my application either.
The rotating exciter is best described by the linked drawing in the first post.

We'll cross the configuration and settings bridge when we get there.

 

[pwrengrds]

In case of a fault what will collapse the generator field if you don't have the discharge resistor? The DC generator isn't going to take back the energy it produced, the excitation system needs to be collapsed in case of faults and without the discharge resistor there is not anywhere for the energy to go.

Ask Basler for a second opinion from one of their senior engineers. I would not recommend getting rid of the field breaker and the discharge resistor.

The shunt exciter field needs to be disconnected to the series field (disconnect the wire between the shunt field and the series field (the DECS will be happier).

David

 

[CurrentKills]

Thanks David.
I was just talking to Basler, and they confirmed that the series field should be bypassed/removed, and I agree that the shunt field should be disconnected from the armature & series field.

In case of fault (resulting in trip), the exciter shunt field will collapse through the DECS output SCRs, with current decaying naturally thanks to the shunt winding's own resistance. Similarly, the main field will decay naturally through its own winding resistance and the resistance of the exciter armature winding. Yes, this decay will probably be slower than with the main field contactor and discharge resistor, but Basler "Senior Application Specialist" says that this natural decay would be acceptably short in this small machine (2MW). That's a bit hand-wavy, but I accept it.

Have you replaced a main field contactor recently?
If so, what was your application and what did you install for a contactor?

Luke

 

[pwrengrds]

Basler's specialist knows what he is talking about and I would accept his recommendations. I haven't had to replace a field breaker, AC is much easier to switch than DC, usually we ended up with a remanufactured field breaker that was questionable.

Basler has some straightforward programming recommendations for the DECS gains need to be installed for a starting point, they work really well. Make sure and get the gain numbers before startup.

David