Substation Battery Charger

[Pam60]

Hi,

I am supposed to work on a substation battery charger schematic and am working on getting some basic conceptual info.

In this system, when station service transformer supply is avilable, the recifier will supply the DC panel but as soon as the AC supply will be off, the batteries will start supplying the DC panel.

or Batteries are always doing the work and the charger is charging them continously.

Could you please advise me how this cycle works. I could not get any literature on google. It is a 125 V DC system.

Thanks

 

[pwrengrds]

The batteries are always on line, if AC is available it charges the batteries as needed and supplies power to the load. If more power is needed it comes from the battery. The battery floats usually and the charger supplies power for the relays and indication lights.

 

[cranky108]

The battery charger can be single phase or three phase, you usually specify this when you order one.
The battery charger also can provide an increased voltage for a limited time (or other means) to prevent sulfination, and equlize the voltage between cells.

Also 125Vdc is a general term where the actual voltage will depend on the number and type of cells. A battery charger will be adjustible or specific to the number of cells, or battery types. This also applies to the alarms on the battery charger.

 

[j3n5]

The charger and batteries are connected in parallel, each with it's own protection. You might need to have a fuse for the batteries (they tend to have high short-circuit power).
The charger needs to be "intelligent" in choosing charging voltage and possibly have some alarm functions to inform the SCADA. (something like this might do it:

http://www.aiabdc.se/eng/products/rectifiers_19.php).

You should consider sizing the rectifier to supply the whole load and charge the batteries from 0 to full (at the same time) within a given time (which depends on your needs).

 

[raithrovers1]

I find that the easiest way to explain how an industrial charger works is to compare it with a laptop computer.
If your laptop is plugged into the AC mains the power supply converts this to a DC voltage (Rectifier). The rectifier is set at a specific voltage to keep the laptop battery charged at a "float" level ie. a voltage that prevents the self discharge of the battery. The rectifier also supllies the power to the laptop components at the same voltage level.
If you switch the mains off, because the battery is always in circuit, it starts to supply the laptop circuitry without any interuption.
Therefore there is no real cycle.
You either have AC power available for your substation charger and it will keep the batteries charged and supply power to your substation loads.
Or, if you lose power, the batteries continue supplying the substation loads without any switching or interruption.

UPS engineer

http://www.powerups.co.uk

 

[Pam60]

Thanks a lot for your input.

 

[deldru]

ECM has a good article on the fundamentals of backup battery systems. You'll have to register to see the article but it does address some of the more important concepts.

http://ecmweb.com/mag/electric_battery_sizing_basics/index.html?smte=wr

Hope this helps

 

[busbar]

Some charger manufacturers supply units with electrolytic capacitors on their DC outputs, and call them “charger/eliminators,” to allow (limited, hopefully) use of the charger as the sole source of DC power.

 

[racobb]

busbar,

You are correct but the user needs to be very careful if the loads are DC micro relays or anything that is sensitive to AC ripple. The charger also uses the battery (great filter) to filter out the AC ripple that can destroy a relay that expects a clean DC supply. Depends on the design of the charger output.

Just a thought!

Alan

 

[cranky108]

I hope most chargers have filtering capacitors!!!

I have seen microprocessor relays misoperate because of excessive ripple current. This is a very good reason to look at the amount of ripple current produced.

Consiter replacing a battery charger at the same time you replace a station battery set. This is important as the electronics don't last forever.

 

[stevenal]

Cranky,

Most chargers do have filtering capacitors, but they also rely on a connected battery to keep the ripple within spec. A battery eliminator filter will keep the ripple within spec with no connected battery. Handy for changing the battery or cells, discharge tests, etc.

 

[racobb]

It's the old get what you pay for. The cheap ones don't have much filtering.

Alan