125VDC Power Distribution 2

[CuriousElectron]

Greetings,

Can someone enlighten me on how does a 125VDC power distribution work with a battery charger and a battery. I'm looking at a single line where the battery is connected to the mains of DC panel and battery charger to the branch breaker of the DC panel. How does the power flow work here - BC feeds the battery and the battery feeds the loads off of the panel? When AC voltage is lot, BC is cut off and battery continues to feed the panel until it reaches the end of its capacity?

I appreciate your feedback.

Thanks,
EE

 

[dpc]

Basically. Normally, the battery floats on the charger output voltage. This will be higher than the battery voltage without the charger. So the power basically flows from the charger to the loads unless load exceeds the charger output capacity. It's dc, so power directly relates to voltage and direction of current flow.

 

[che12345]

Dear Mr CuriousElectron

I presumed the problem is you are looking at the (single line) with some simplification or may had omitted some details which are considered as (understood). The omission is that the battery charger [input] is connected to the ac source; which from your description, (is not? ) shown in the single-line drawing.
The system works as follow:
a) When the ac is available, it is connected to the charger (input). The charger (output) is connected to the battery and the dc line. The charger [output voltage] is adjusted to be > the battery voltage; so that it [charges] the battery (continuously) and also feeds the dc load.
b) When ac fails/not available, there is no ac supply to the charger input, therefore no output from it. The battery [discharges] into the dc line (automatically) without any disruption. However, the dc line [voltage is dependent on the charge] which was stored in the battery when the ac was available, i.e. before the failure. The dc line maintained the supply without any interruption, when the ac failed or upon resumption. The dc line voltage will drop gradually, depending on the battery stored charge and the load.
Note: The battery charger is [always ac operated]. There is [no battery] inside the charger. It basically comprising of a step-down transformer, rectifier with smoothing capacitors and charging voltage sensor with current limit circuitries. It does not invert the dc voltage to ac.
c) Upon ac resumption, with ac available at the charger [input]; it commences to [re-charge] the battery and feeds the dc line (automatically). There is no disruption on the dc line.

Che Kuan Yau (Singapore)

 

[Sn00ze]

Curious Electron; Che12345 covered the high level stuff really well. When you get into your design, I want to mention that you should request the DC breaker that takes in the Rectifier's output to come with a Trip and Open indicator Aux relay.

You will need to know when this breaker trips due to a fault and/or is open. Otherwise you risk running on batteries inadvertently, due to the nature of UPS systems.

 

[che12345]

Dear MrCuriousElectron

Thank you Mr Sn00ze for the very relevant and valid advice.

An alternative is to install a [dc under-voltage] relay that monitors the dc line [voltage] continuously. This relay may be set at say e.g. 0.95 = (95%) of the line voltage.
Note: There are basically two different types of dc under-voltage relay:
a) with [dc auxiliary supply] , sensing the dc voltage. Use the [NC contact] to activate the alarm. When the [dc aux supply] > say 0.85 Uaux + the line voltage Uline > Uset voltage, the NC contact changes from "close" to "open" . Advantage: easy application and easy to understand the functionality. Disadvantage: The NC contact remain "close" when the dc Uaux < say 0.8 Ur.
b) with [ac auxiliary supply], sensing the dc voltage. Use the [NC contact]to activate the alarm.
When ac Uaux is available + Uline > Uset voltage, the NC contact "opens" [instantaneously]. With ac Uaux available but U line < U set voltage, the contact is in "close" status. Advantage/disadvantage: This functionality is needed under certain circumstances.

Che Kuan Yau (Singapore)

 

[Sn00ze]

Mr. che12345,

Some IED's come equipped with a DC monitor for your batteries an under utilized feature, not sure why.

Please see attached.
It may be a cheaper option if you are already using a relay equipped with it. This particular one is from a SEL-421 but i believe most SEL have it.

 

[che12345]

Dear MrCuriousElectron

Thank you Mr Sn00ze for your learned advice.
It is essential to have some form of [under-voltage] monitoring to activate the alarm when the dc voltage goes below the desired value, due to any unforeseen circumstances.

Che Kuan Yau (Singapore)