250 VDC system and Breaker Ratings 1

[basilasq]

We have a substation DC loads supplied via a 250 VDC system. The charger float voltage is actually 270-280 VDC. The question that came up was that all the standard breakers are rated for 250 VDC, and none of the application engineers from Schneider, Eaton, ABB etc. are willing to state that the breakers can operate at the float voltage which is about 10% over the nominal rating. The other option is to spec 600V breakers which is impractical for the whole system given the amount of DC branches and loads.

Is there an overload rating of the breakers at a lower kA value that they are able to operate?

 

[davidbeach]

Never worked with 250VDC, but that's twice the 125VDC that I'm used to and your float voltages are twice what I see on the 125VDC system (125V means we've lost station service and the charger has shut down). Never had any of a myriad of breakers have an issue with 135+ VDC. No idea why the whole range of stuff for 250 can't be twice what it is for 125. Maybe the salesmen just don't know what 250VDC (or maybe even 125VDC) actually means.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[che12345]

Dear Mr. basilasq (Electrical)(OP)16 Dec 22 00:51
"#1.... a substation DC loads supplied via a 250 VDC system. The charger float voltage is actually 270-280 VDC..... all the standard breakers are rated for 250 VDC, and none of the application engineers from .... are willing to state that the breakers can operate at the float voltage which is about 10% over the nominal rating....."
1. In most cases, the same DC CB rated for 250Vdc can be used up to 600Vdc.
Attention: (a) Check the DC source whether is earthed at one pole, non-earthed or center-point earthed.
(b) Consider the use of 3-pole CB ; with two-poles in series. Or, the use of 4-pole CB with varies possible connections. etc. etc.
(c) With the same pole connection, the kA rating at 600Vdc is usually/would be lower than the kA rating at 250Vdc.
"#2. ...... The other option is to spec 600V breakers which is impractical for the whole system given the amount of DC branches and loads."
2. In most cases, the same 250Vdc CB can be used up to 600Vdc. See above 1. Therefore, you do NOT have to specify a 600Vdc CB.
As an example, XXX manufacturer CB type T1B 160 rated 10-100Adc, 16kA (2-pole) 250Vdc is rated 16kAdc 600Vdc (3-pole). There are other types in the series > 16kA at 600Vdc .
3. BTW: These information may not be published in the general catalogue.
Che Kuan Yau (Singapore)

 

[basilasq]

Thank you @davidbeach and @che12345 for the helpful replies!

 

[stevenal]

The DC voltage for breakers can be found in IEEE C37.06. For closing, energy storage, and auxiliary functions for Class S1 breakers: 200-280. For Class S2 and those rated 100 kV and above: 180-280. For opening of all types: 140-280. Keep the float at 280 or below, and all should be well.

The row for 125 DC is exactly half that of 250.

 

[davidbeach]

I think that sevenal and I both read the question the same way, but I wonder the questions isn't about the breakers protecting the 250VDC system rather than the breakers being powered by the 250VDC system.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dpc]

I think they are concerned about the dc breakers in the dc distribution system. I'd try to contact the factory directly. But we use to have similar issues with devices rated up to 125 or 120 V dc on a battery system that floated at 130, nevermind equalization. Indicating lights were a particular issue. The breakers are probably OK and built for 600 V, but good luck getting the supplier to endorse use that exceeds the rated values.

 

[basilasq]

The DC voltage for breakers can be found in IEEE C37.06. For closing, energy storage, and auxiliary functions for Class S1 breakers: 200-280. For Class S2 and those rated 100 kV and above: 180-280. For opening of all types: 140-280. Keep the float at 280 or below, and all should be well.

The row for 125 DC is exactly half that of 250.

Thansk for the response. However, the question was not about control voltages for HV breakers but the DC breakers used in the 250VDC distribution system.

The breakers are probably OK and built for 600 V, but good luck getting the supplier to endorse use that exceeds the rated values.

Exactly the feedback we got. Eaton stated that they'd be willing to do type tests for us but at an exorbitant cost and lead time.

 

[dpc]

Just be aware that the dc interrupting and switching current ratings will be much lower than the ac ratings.

 

[jraef]

DC switching ratings are all about arc extinguishing capabilities because DC arcs are sustained much longer than AC arcs, therefore the DC ratings and AC ratings are never the same or even similar. So if a breaker is tested and listed for 250VDC, I would take that as it is stated.

In most cases, the same 250Vdc CB can be used up to 600Vdc. See above 1. Therefore, you do NOT have to specify a 600Vdc CB.

I would qualify that statement. That would only be the case if the base design of the contacts was 600VDC, because using them at a lower DC voltage is never a problem. OR it IS possible that, if you have a multi pole breaker and you loop one pole back through the 3rd, so that one pole of the DC is going through multiple sets of contacts in series, THEN this can often allow for a higher DC voltage. The air gap is considered to be cumulative, so two or three air gaps in series is actually better than the rating of one air gap alone. So a number of circuit breakers with DC voltage ratings will have this higher rating, again so long as you can series once side of the DC.
Case in point:

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden