Emergency generator cramking amps

[raithrovers1]

I am on site and they have a Em Gen module supplied by Aggreko. It is a Cummins genset 835kW. They went for a start a few days ago and it wouldn't start.
I am on site to maintain their UPS systems and they asked if I would have a look.
The batteries were at 24Vdc (should have been between 27.2V to 27.6V wet lead acid). There are two batteries strings which are selected manually for duty start. The battery is nominal 24V with 2 x 12V 900CCA batteries.
There are two 24V 35A chargers each supplying their dedicated battery. Both of the charger healthy lights were on. The problem was found to be the charger output fuse being ruptured on both chargers. This output fuse is rated at 100A.
The fuses were replaced and the chargers started to supply the battery with current that was limited by the charger at approximately 28A by the charger current limit. The batteries were left for a day to charge up.
The following day both battery strings were checked and found to be at 27.3Vdc.
I connected a scope on the output fuse, monitoring voltage and current. The genset was then started. Voltage at charger dipped to 14Vdc, current was measured at 74A for 3 seconds. The genset started without any problems.
The scope was then attached to the battery and the same test was performed. Battery voltage dipped to 17V and current was measured at 1500A! On no occassion did the fuse blow.
It was then discovered speaking to other on site personnel that the fuses blowing was a reoccuring issue.
Therefore I have two questions:
1. Should a genset this size be taking 1500A to start?
2. Has anyone had any experience with genset chargers overshooting the current limit on genset start up and blowing charger output fuses?
Thanks for any help.

UPS engineer

http://www.powerups.co.uk

 

[Hoxton]

1500A does not seem out of the range, on a gas engine at 1000kw (1200kva), not a cummins, I have

Starter 15kw (that is the continuous rating)
Battery capacity 4 x 143Ah
Cranking current at -18C 1140A
Short circuit current at +20C 3500A

Remember that a gas engine has a lower compression ratio than a diesel engine.

 

[catserveng]

Yoru cranking current and voltage drop seem normal for a diesel genset engine of that size.

I have run into many sites where the charger output fuse can fail during engine crank. Some engine vendors had a cutout circuit for the battery charger to prevent this type problem. Also, where are the battery charger output lead connected? You may find you have less problems if the output leads from the charger are hooked up at the battery terminals instead of the starter terminals.

Hope that helps.

 

[nawao]

Be sure that the two batteries strings will never be in parallel, even momentary, when are
selected manually.
Some switches have three positions: Bat1 - Bat2 - All.
Paralleling batteries shorten their lives and could activate charger’s protections.

 

[raithrovers1]

Still trying to get decent drawings.
Looking at the setup at the moment, I don't think the charger cables go to the battery terminals. They are connected eleswhere. I agree that moving them diectly on to the battery terminals would help.
The chargers do have a contact on the mains input side that opens when the genset is up and running. It takes about three seconds to operate after the start is initiated. Don't know if it was designed for charger protection or to disconnect charger when alternator is running.

UPS engineer

http://www.powerups.co.uk