DC load calculations of MV voltage breaker

[budhiman]

Hello all,

I was wondering if any one has any thoughts on DC load calculation for MV switchgear (125Vdc control power). This is to design the battery. Will the spring charging motor in the breaker considered as an intermittent source or continuous. Continuous for only Amps calculation. But it is a non continuous load.

 

[davidbeach]

Better look to see if it is even a DC load; it might be an AC load. But even if DC; if you aren't auto reclosing it may not make much difference. Look at the overall sequence while on battery without a charger, do you really need to recharge a spring?

 

[GroovyGuy]

Hi Bud,
One thing that I have noticed wrt battery sizing, for switchgear loads, is how the CB charging motor has little impact on the battery size. While it is true that the charging motors are the largest single load on the battery, but they operate for such a short time (ie 6A for 5s, each charge cycle). The continuous loads imposed by indicating lights, relays, and other continuous loads tend to make up 90% of the amp-hour rating of a battery.

Refer to: IEEE Standard # 485-2010 - IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications

Regards,
GG

"Wish I didn't know now what I didn't know then." -- Bob Seger

 

[krisys]

budhiman,

Being an Indian, I liked your tag name "budhiman" !
As per the IEEE-485, you have to consider a rating factor to compensate the high discharge for a short duration. There are two types of factore namely Kt and Rt. You can use either of these.
I found Kt factor is more intuitive than Rt and I use Kt only. I never understood the Rt factor.
Refer any battery catalogue for the Kt factor.

budhiman, hope it is helpful to you!

 

[Sn00ze]

We don't really take the Spring charge into consideration for DC Calculations. Instead we use the sum of the Trip Coils and motors running during the projected worst case scenario along with the base load. This ultimate current is then used to calculate battery size, and then in turn, battery charger size.