Battery charger operation philisophy

[smnellore]

We have two battery chargers connected in parallel feeding to a common battery bank. Please refer to battery charger SLD attached.Scenario-1 is presently using in our company.But with the present scheme loads are not sharing equally one charger is always taking full load with the other charger output current is zero. The different operating philosophys are explained with pro and cons. Kindly give your valuble feedbacks on these scenarios and which is the best.

SCENARIO-1:

Current operation philosophy is :

· To keep the two isolators A & B Closed

· To keep the load isolators C & D Closed

· To keep the bus coupling isolator E Open.

With the above scenario :

· The chargers are operating in parallel to the battery.

· But the loads are not shared (as there is no communication between the two). Thus one which has a slightly higher rectifier output voltage, reverse bias the output diode on the other charger and takes over the entire load.

· The battery current limit protection would not be active (as both chargers acting in parallel will double this limit).

SCENARIO-2:

Same connection (Isolator positions) as per current operation philosophy mentioned above.

· But it is designed with the objective that ONLY ONE CHARGER WILL BE TAKING THE FULL LOAD, while the other will just remain a HOT STANDBY. (i.e Sharing of load is not envisaged) .

· So in effect one charger will feed the battery and the protection will be taken care by the same.

SCENARIO-3:

Another possible alternate operation scenario :

· To keep either of two isolators A OR B open (So battery charged by one charger)

· To keep the load isolators C & D Closed

· To keep the bus coupling isolator E Closed.

With the above scenario :

· The battery is charged by one charger.

· Loads are shared by both chargers (though may not be in equal proportion).

· The battery current limit protection would be active (as single charger would be charging it).

Disadvantage :

· The short-circuit protection for any downstream (at the load side) fault could impact both chargers

 

[ruggedscot]

Loking at your diagram the DC bus should be kept isolated from looking at it. The way its configured screams out n+1 and having the two supplies hooked up would concern me.

You will never get the chargers to share the charge current as its a dc system and the dc voltage dictates what supply supplies the current.

Id opereate the system with either A or B closed but not both. Id have both C and D closed and E open. This keeps the supplies seperate and maintains n+1 operation overall. The battery is there to provide back up and it doesnt matter what charger is being used - rotate them if you like one month on one side and change over or install another battery bank and hook it up to the other string and take away the link between A and B

rugged

 

[stevenal]

I don't see how 2 differs from 1.

Number 3 is no different than 1, simply a longer route to the battery for one charger. Both chargers feed battery and load in parallel, with one charger hogging the combined load.

I disagree with ruggedscot's suggestion. If (A xor B) and E are open, bus B or C will run without battery backup and high ripple.

 

[smnellore]

I agree with stevenal. My concern is if the batteries are discharged due to some reason (plant blackout) during restoration both the battery chargers try to send the current in to the battery (goes to float charging if boost charge not selected )and it may overcharge and damage the battery.

 

[burnt2x]

I maybe OT but let me ask:
Why did it have to be a two-on-one (charger vs. battery bank)? Ever since, I knew only of one-on-one setups, with provision for paralleling should one charger fail! Did you loose one battery bank in your case?
BTW, you can always trim any rectifier output to equitably share charging amps between paralled chargers.
IMHO, batteries are some of the special equipment in a plant and should always be taken cared of with utmost priority. Losing batteries is just like losing control. During blackouts, restoration of power to rectifiers is the first order you get. That way your fear of discharging you batteries will never happen. Just remember that it's not good practice to discharge batteries down to 75% of charge .
I hope you source your charger supply from upstream of your service with a transfer switch to your emergency(black start) generator!

 

[smnellore]

Burnt 2x
1. Why did it have to be a two-on-one (charger vs. battery bank ??? Low cost.
2. BTW, you can always trim any rectifier output to equitably share charging amps between paralled chargers/// The loads are not shared as there is no communication between the two. Thus one which has a slightly higher rectifier output voltage, reverse bias the output diode on the other charger and takes over the entire load.

3.Just remember that it's not good practice to discharge batteries down to 75% of charge ??
How can you control this ?
4. I hope you source your charger supply from upstream of your service with a transfer switch to your emergency(black start) generator!
Yes .

 

[stevenal]

A charger maintains the voltage, supplying whatever current is needed up to to its capacity. No overcharging should occur, as long as float and equalize set points are correct. If one hogs the load during the normal float condition, I expect it will continue to hog it during the recharge.

In answer to number 3: Size your batteries to the size of the load and expected outage (response time to get to transfer switch and generator). Include a worse case event at the end of this discharge, such as a bus trip. Apply factors for load growth, battery aging, temperature, etc.

 

[smnellore]

Stevenal,
For example if the battery is rated for 100A in float charge .condider the case of discharge and restoration each charger try to send 100A to the battery i.e total 200A flows in the battery.In this case how the battery will be protected?? How the charger protections will be set it will be based on parallel operation or on individual basis??

 

[ruggedscot]

Hi

It was just my take on the system look at it through a n+1 scenario. Id say the power supplies are low ripple designed for this sort of set up. Id hate to see the two sides pulled together as that asks for trouble - strings need to be seperated ! the battery is that not there for back up functions ? Also if one side fails would you not close up E to link them together as a temporary solution.

Having two chargers linked may alter the protection levels and also upset any charger controllers. just my 2 cents worth.

 

[smnellore]

Based on above discussions it seems that the existing operation scenario is not acceptable. For all this is the approved philosophy for one of the siemens power plant. I dont know the reason for siemens philosophy. Let us see other forum experts views on this topic

 

[stevenal]

Usually the voltage is specified, and the current at that voltage is then limited by battery resistance.

 

[ScottyUK]

If one charger hogs the load... so what? The battery is charged, the load is supplied, the other charger is available to accept load if the other fails.

If the battery is well discharged then the chargers will work together in current limit to recharge the battery. Once they enter float mode one will hog the load. If one charger hogs the load... so what? The battery is charged, the load is supplied, the other charger is available to accept load if the other fails.

Parallel chargers serving a common battery have been used for years without any problem. Are some people making this more complicated than it needs to be?


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Im fully agree with Scotty.
It's typical application for lot of small substations.
Of course better is two battery banks.
And of course very important Stevenal's point:
"Include a worse case event at the end of this discharge, such as a bus trip".
Regards.
Slava

 

[smnellore]

Scotty UK
"If the battery is well discharged then the chargers will work together in current limit to recharge the battery"

Is there any setting will be there for current limit?If so what setting should adopt it should be 100% of battery capacity (considering one battery charger) or 50% (considering two battery chargers feeding) for .Is it possible to get information on working of Battery charger current limiter.
Thanks for all your feedbacks.

 

[ScottyUK]

Current limit setting is an awkward question. Ideally the combined output of the chargers would be no greater than the maximum acceptable charge rate for the cells. Obviously this would mean an extended recharge time with only one charger available. In most circumstances I would set the current limit of each charger to 50% to maximise the battery life. You need to make a judgement whether getting the battery back to full charge quickly is worth sacrificing some battery life.

On the old fashioned chargers based on analogue electronics it should be fairly simple to arrange a relay to switch between two current limit values. Most chargers have a 'charger fail' alarm relay: the 'charger fail' relay of charger 1 would modify the current limit of charger 2 and vice versa. It is open to question whether the improvement worth the effort though.


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

When you wrote:

"Scenario-1 is presently using in our ompany"

"SCENARIO-1:

Current operation philosophy is :

· To keep the two isolators A & B Closed

· To keep the load isolators C & D Closed

· To keep the bus coupling isolator E Open. "

It is clear your two DC loads are supplied by both Chargers A and B. But you said;

"The loads are not shared as there is no communication between the two."

(in your 5 Sep 08 5:35 post!)

Closing "isolators" A,B,C, and D makes the loads being at the same voltage as the chargers A & B, hence they are connected and can be shared equitably if you tweak the rectifier volt-adjust knobs of each rectifier!
We all agree that if you do not adjust the rectifier settings, the one charger hugging the whole load will continue to hug the load!