125V DC circuit breaker

[odlanor]

DC voltage for power plant auxiliary system is 125 V DC.
They use molded case 690V-AC circuit breaker Siemens Sentron 3VL27 Term.: 125-160A; Mag.: 800-1600A.
I guess it results in a bad overcurrent protection coordination.
-Is there 125V DC circuit breaker?
- Do you know any reference for DC overcurrent protection coordination ?

 

[jraef]

That breaker is rated for up to 500VDC. That has nothing to do with the corrdination. Separate issue.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[odlanor]

let me explain it another way.
Was purchased AC circuit breakers to be applied to the system 125 V DC. From the viewpoint of interruption fully meets the breaker.
From the standpoint of coordination of overcurrent protection, the triggers(relays?) attached to the AC circuit breakers had a bad coordination. Hence the questions:

-Is there 125V DC circuit breaker?
- Do you know any reference for overcurrent protection coordination or DC?

 

[gvasiliou]

Since the breaker you have can support up to 500V DC, it is ok for you. You don't need a specific breaker for 125 V DC.

I guess what you really want in your case is a different trip unit to be fitted in the circuit breaker you already have.
If i'm not wrong almost every circuit breaker gives you the option to select the "tripping unit characteristics".

I think that if you check with your local Siemens supplier (or Siemens web site) it is quite possible to find a tripping unit for the particular circuit breaker suitable for DC systems so you can just replace tripping unit and you are fine.

 

[jraef]

The tripping units don't know from AC or DC, they are based strictly on current.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jghrist]

Microprocessor overcurrent relays (and I assume LV breaker trip units) normally use digital filters to extract the fundamental component of the sampled current values, so the dc may be filtered out. You need to check with Siemens to see how the trip unit responds to dc current.

 

[JG2828]

If you wanted to get all fancy you could install a shunt resistor and use the ABB 76T relay or similar to trip the breaker.

 

[DTR2011]

What part of the plant auxiliary are there problems? Is this a main breaker (MCB), feeding smaller (~10-15A) DC loads? I would suppose that Siemens offers DC panelboards with proper coordination for DC loads - check with them.

With the smaller size breaker you have mentioned, I don't think a lot of the options mentioned above will be available as retrofit.

 

[jraef]

That Siemens breaker is a simple bimetal thermal and magnetic trip, nothing fancy.

I too am curious about the need for coordination on the DC side of station service power. Coordination with what?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[gvasiliou]

Stupid Question :
Since they have 690V-AC Circuit Breaker i assume it is 3 or 4 pole.
No matter what is the trip unit, how a 3 pole CB deals with two poles of DC systems...?

 

[odlanor]

What´s happened if a DC fault at load, CB at load do not open ?
It must operate CB feeder of DC subpanel connected to that load.

What´s happened if a DC fault at subpanel, CB at subpanel do not open ?
It must operate CB feeder of DC panel connected to that subpanel

 

[gvasiliou]

Hi odlanor,

Did you check with Siemens the possibility to replace the trip unit?

 

[odlanor]

not yet. They did not want answer my question about, because I can change assembled equipment . Now a administrative problem beyond my decision.

 

[odlanor]

jraef (Electrical)
"The tripping units don't know from AC or DC, they are based strictly on current "

A current 100A DC is completely different of 100A AC.

 

[sibeen]

Microprocessor overcurrent relays (and I assume LV breaker trip units) normally use digital filters to extract the fundamental component of the sampled current values, so the dc may be filtered out.

That can certainly be the case. Many years ago I was working for a UPS manufacturer. A circuit breaker was installed in the battery feed, which relied upon the magnetic trip to operate in the case that there was an internal fault in the UPS.

The circuit breaker manufacturer changed over to a microprocessor based trip unit and no-one within our company questioned the change. A year or so later we had a fault in a unit with a micro trip unit and the result was a complete system that needed to be replaced and much embarrassment and head shaking within the engineering section of the company.

 

[jraef]

"A current 100A DC is completely different of 100A AC."

Not sure you understand or maybe I'm the one not following your logic here. The EFFECT of 100A AC current and 100A DC current on a bi-metal sensing element in a protective device is the same. The magnetic field induced in a magnetic trip coil is the same at 1000A AC or 1000A DC, in fact when considering an AC mag trip coil, the effect on the coil is virtually DC anyway.

Electronic trips, I agree are totally different. But THIS circuit breaker does not have electronic trips, it has a bi-metal thermal trip and a plunger type magnetic trip coil. The reaction to AC or DC current is virtually the same and more to the point, there would be no "coordination" (whatever you mean by that) issues to consider that I can think of.

The term "coordination" when used in the context of circuit breakers has to do with the timing of trip settings so that any fault is cleared at the lowest possible level. In a 125VDC aux. power system within a power plant, how many sub-feed breakers are you going to have below a 160A breaker and how many levels of Mains are you going to have above it? I'd be willing to bet you have no more than 10 little MCBs down stream of this breaker, and maybe ONE breaker up stream. I can't think of a single MCB that would hold in longer than this breaker, and if you have one up stream breaker, THAT is the one to get careful on with regards to coordinating with this breaker. I just don't see this as being all that consequential in the grand scheme of an aux. power system.

But oh well, you seem bent on making this a bigger deal than it likely deserves and yet you can't talk to Siemens. Good luck.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[odlanor]

jraef,
I really liked your comment. It's constructive criticism!
Iwish you reformulate the text in another way, to my understanding:

I'd be willing to bet you have no more than 10 little MCBs down stream of this breaker, and maybe ONE breaker up stream. I can't think of a single MCB that would hold in longer than this breaker, and if you have one up stream breaker, THAT is the one to get careful on with regards to coordinating with this breaker.