Power Engineering

[Jaimegu]

Hi there!

I had a power outage a couple of weeks ago and I had an eye on the 4 rectifiers (MPR25)... They went over 28Amps each one and the power outage lasted only 12 minutes.
The battery backup is designed for 2 hours.
The 8 Batteries (2 banks of 12V-75AH ea.) are OK as they were tested the same week.
Floating voltage is 54V and consumption in normal condition is 54Amp (just coincidence!).

Is it normal that the MPR25s supply 28Amps (even for short periods) or can they get damaged?

Do I have to buy another MPR25 just for these events or our design is correct?

Do you think there is a problem with design of the Nortel rectifier in terms of battery charger?

TIA

Jaime G.

 

[johnpoole]

sounds like your system is pushing the limit for bower consumption.. for longer power outages you might want to double your battery bank and look at another mpr25.. another concern you might want to address is one i have used on larger switches, for longer power outages i have powered down the 90v power supplies for my analog stations, this will make a 2 hour backup battery last 4+.. not always a solution but in some cases analogs are faxes and modems mostly, and can be used outgoing during a crisis with no loss of service that can not be cost justified.. here at the hosp we have generators that take over after 1 minute. analog phones here are used for life support functions so dropping ringing is not an option.. at a large insurance company, it's a normal routine.. as long as the inbound acd's stay up...

john poole
bellsouth business
columbia,sc

 

[Jaimegu]

Thanks John,

Great tip on shutting off Ring Gens! I guess it could help despite of having 90% on digital sets.

I also think I didn't explain it well: We had to change a breaker so we needed the 81C to operate on batteries for a while. When the power came back (after 12 minutes), the rectifiers offered 28 Amps each, and started going down to in normal level of 15 Amps each. I find this peak a potential trouble but more on the side of battery charger design than power engineering, since we use the rectifiers just in their 60% in normal conditions.

 

[johnpoole]

your surge load after power restore sounds normal, the float and equalize adjustment might need trimed but how fast did the rect. come back to normal readings.. if that value was less then 3 times the length of your power outage, your well within your engineered levels

john poole
bellsouth business
columbia,sc

 

[AJ9U]

The fact that the MPR25's went to 28 amps does not mean there is a problem with them. Nortel specs state that the maximum output of an MPR25 is 25.5 to 30 amps, so 28 amps is within specs. There is not a design flaw, just normal operation as designed.

If you have few outages (and therefore can live with long recharge times, say in excess of 12 or 24 hours) and the batteries are VRLA, multiply your equipment total load times 1.15, and that will give you current output needed from your rectifiers. So, with a 54 amp load, rectifier output should be 62 amps, which 3 modules handles fine.

If you lose one rectifier, can the remaining ones carry 100% of the load? If not, you need to add more. This is called "N+1 redundancy." With N+1 redundancy, and loads as described above, you need 4 modules.

Bob