Battery system Sizing 4

[kookypedia]

Guys,
I am sizing battery systems for an LNG plant. We are in the preliminary stage and we have not bought LV and MV breakers.
I am taking the conservative approach of considering 1 trip coil + 1 close coil for all LV and MV breakers shown in overall One line diagram.
Do you think this is a reasonable design or I am over-sizing the battery system?!

 

[davidbeach]

I don't do battery sizing calcs so I may not have all the factors but tripping comes at the end of some duration without the chargers. All the indicators, meter power supplies, relay power supplies, SCADA system power supplies, control room DC lights, etc. all draw down the batteries for 24 hours or longer. Then you need to be able to trip some number of breakers, but not necessarily all. There's no event that needs to trip everything.

Once you've cleared that worst case fault, a bus fault somewhere, you still have all the continuous DC loads and may still need to clear more further up stream. Eventually you'll need to be able to close something to get AC station service restored to the chargers. I see no reason to need to close everything on the battery without the chargers. Also, keep in mind that trips can be simultaneous, but closes will almost always be sequential.

 

[Sn00ze]

In the DC studies i've done. The client requires 8 hours to be considered for the battery banks. (I imagine 24 hours would require a lot more batteries and a bigger charger, which in turn would increase your AC service).

Depending on what your client wants, we are asked to consider worst fault scenario during 1 minute. Usually a main TX or Gen fault. which includes breakers and air breaks to isolate the fault/bus.
Then for 478 minutes we are asked to consider steady load. These are the indicators that DavidBeach refers to, relays, SCADA etc.
then on the last minute we are asked to consider restoring the breaker (with the biggest closing current and motor starting current) at a time.

Again, these would change depending on your client. I remember one time we ha a fault during the weekend and we couldnt get a crew until monday. We had to have a guy run a gas generator to keep the system going. Poor guy spent the night in his truck. hahah

 

[kookypedia]

24 looks too much, my Dc system requires 4 hours lifetime (24 hours too charge from complete discharged case).

 

[bacon4life]

Counting the breaker coils for doing voltage drop calculations is the easy part, but the total energy consumed by trip coils is very small. The real driver for the overall energy capacity of the battery system is the 8-24 hours or more of consumption by all the other miscellaneous equipment David mentioned. Be sure to leave capacity for future DC powered equipment as it is unlikely the battery sizing will be reviewed each time a new device is added.

Also, IEEE 485 has good advice.

 

[racookpe1978]

What battery degradation factor are you assuming?

(Over time, all batteries lose capacity to deliver power, so you need to assume some loss factor (or some lifetime percent of use) that reduces your "initial purchase" capacity. )