Battery replacement in a substation 1

[wolfie1a]

A few years back we changed out our old wet cells batteries for the new Gel cells. Making a long story short we need to replace the Gel cells. I’m looking for guidance / procedures on how to safely change these one-line. Any help would be greatly appreciated.

 

[crshears]

Um...did you swap the wet cells for gels by taking the facility out of service? If so, can you do it the same way this time?

If the last time you did a 'hot swap,' what's different this time?

I'm trying to understand the question...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[dpc]

Most battery chargers can operate without a battery load. In this case, you can run the dc system directly off the battery chargers while the batteries are replaced. Just verify that the charger is designed for this.

 

[crshears]

Hi dpc, fair enough; however this may not satisfy redundancy and/or carryover criteria when planning the work, depending on the application.

The general practice for this type of outage in my utility is to bring in a temporary battery bank to provide the necessary degree of uninterruptibility. The Ampere-hour rating of the temporary bank is some fraction of the normal bank's rating, as determined by the Engineering Department taking into consideration the amount of load to be carried and the criticality of that location to the grid. The prime concern is of course that the temp bank last long enough to ride through contingencies until such time as maintenance crews arrive to take appropriate remedial actions.

Using a special portable DC switching tool, the temp bank is paralleled with the existing one, then the bank to be replaced is removed from service, with the process reversed once replacement has been completed.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wolfie1a]

Sorry I did not provide the background story. The wet cells were changed out when we had a plant 10 year power outage (several years ago). The Gel cells batteries will not make it to the next power outage and taking down the substation is not an option. As part of the outage we also upgraded the protective relays… powered off the battery string. The main problem opening the battery string to replace them is the charger voltage will go high. We found the power supplies in many of the relays would not take the elevated charger voltage and burnt out. In doing some web searches I found some documents that referencing paralleling batteries as suggested above. I’ve also seen to discussion on using a diode in replacing the batteries. As stated earlier.. looking for some guidance on how substation batteries should be on-line.

 

[racookpe1978]

Parallel the dc charging system with a temporary cell assembly (partial load capable) on a semi-trailor that you can pull to the site, hook up, then move to the next sub-station or back to the maintenance shop for emergencies.

 

[DTR2011]

"Most battery chargers can operate without a battery load. In this case, you can run the dc system directly off the battery chargers while the batteries are replaced. Just verify that the charger is designed for this."

This is a true statement, however to stress the importance of this, I will share a horror story.

A few years ago, a plant was changing out a bad cell in a string. Rather than using a temporary battery in parallel with the old one, a decision was made to turn off the DC feed to the batteries and run off the charger (without knowing the details of the charger). Battery chargers can be ordered with the proper filtering to accommodate such situations, but in this case there was not such an option on the charger (sometimes called a battery eliminator, or a filter, etc).

When the batteries were removed( via battery breaker), 8 relays immediately went up in smoke and the plant tripped. Over $250k of product was lost. This battery charger did not have the proper filtering (that the batteries provide when connected). 5 of the relays were static devices and not readily available. The 3 SEL 500 series relays were common enough that power supplies were able to be swapped out from other, similar units.

To be clear on the procedure (and a simple automotive battery will do for a single cell change out, jumper cables (or other suitable) are placed in parallel with the battery to be replaced. After it is established that all connections are secure, the bad battery can be removed / replaced. Un install in reverse order.

 

[crshears]

A different specially-made live switching tool than the one mentioned above can be fabricated to safely accommodate the replacement of a single cell, provided the connected equipment can tolerate the slight voltage drop involved. The device consists of a selector switch, a resistor, and the necessary load-current-rated bus bars. A picture and description is buried within some old [read "almost ancient"] training materials I have at home, having rescued same from being tossed in the recycling bin years ago. If I can find it, I'll scan the stuff and post it here.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]