Substation battery nominal voltage 1

[Thedroid]

I'm installing a new battery next week in our main substation, and wanted some advice. Load will be 12 GE magneblast breakers. Nominal voltage is 125, closing coil is 90-130 95A and the trip coilis 70-140 6A. The system I'm replacing consisted of 54 cells but looking at the PO 60 were purchased. These batteries have been in service for along time and have started failing. I have a new battery consisting of 60 cells that I intend to install and float at 132-135. All of our relays are electro-mechanical, and the only thing the batteries will supply will be the indicator lights, breakers, and lifting/lowering mechanism. I don't feel that the extra 5V on float will cause any problems, and the voltage drop will probably cancel it. Is this a common concern? I've have read that some people leave out a cell over voltage concerns, but I'm not sure if this applies in this case. I can't find any information or drawings on the original system, other 125 VDC Bus on the swgr scematics.

 

[oldfieldguy]

It was not an uncommon practice to pull a bad cell or two from a string if they failed. That's how yours likely got down to 54 cells. However, most folks replaced the failed cell at the earliest opportunity.

125 volt banks are commonly 60 lead-acid cells and normally run at the voltages you describe. Consult the battery manufacturer literature for actual float and equalize voltages, but your numbers are in the ballpark.



old field guy

 

[Thedroid]

Thanks, that's what I was thinking. I've gone through the literature, and have the specs just wanted to hear that its common. Under what circumstance would you purposely omit a cell or two? I would guess if you were going to have to equalize online that might put the voltage a little high. I'm using C&D DCU-9 Lead Calcium batteries. It appears that these do not require an equalize charge.

 

[ScottyUK]

110V nominal systems typically float at about 124V. These are usually configured as 55x 2V cells but if the battery is made up of 6V blocs then 9x 6V blocs - 54 cells - is quite likely too. This isn't an especially unusual configuration with the relatively small cell sizes used in distribution substation tripping batteries. Transmission substations typically use individual 2V cells but are much larger in capacity.


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

 

[davidbeach]

Scotty, North American practice seems to be 60 cells, called 125VDC, but actually closer to 135V.

 

[ScottyUK]

Interesting! We have spec'd 125V DC relays because they last longer on 110V DC. At least one manufacturer doesn't have a wide enough tolerance to accomodate float voltage. Kinda lax of the manufacturer because they know - or should have a damned good idea - what power source they are going to be fed from.


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

 

[stevenal]

Per IEEE C37.06 your maximum voltage for 125 nominal is 140. Unless you plan to disconnect your battery for equalization, I would compare 140 to your EQ voltage.

More cells will give you more run time for a given ampacity to the minimum 70 V. We use 60 cells, which puts the lower end of the manufacturer's suggested equalization voltage right at 140.

 

[Thedroid]

We installed the batteries today without any problems. Tommorow we will connect them to the charger and let the charge for 72hrs. After that a load test, and we can hook them back up to the swgr. I believe the instructions said that the Lead/Calcium didn't need to be equalized, and also that the initial charge would occur at the float voltage. I'll double check tomorrow.

 

[mmt019]

Be careful that when equalizing your batteries that your voltage does not exceed the operating voltage of your components that are on battery power. I believe 60 cells is a common cell count at 125VDC. Sounds like you will be ok but check your max. equalization voltage.

 

[stevenal]

Equalization is not performed routinely, it is performed when cell voltages or specific gravities are not equal (not within battery manufacturer's acceptable deviation). It is also performed whenever cells are discharged to below the minimum (generally 2.13 V). See IEEE 450. When calling regarding a warranty replacement, they will ask you to equalize if you have not already done so. Unless you can disconnect your devices (why would you have battery backup if you can disconnect them?) they will need to be able to withstand EQ voltage.