Hello Automation List Subscribers,

First, I'd like to introduce myself. My name is Enoch Wu. I work for a municipal electric utility. Presently one of my projects is
"Installation of Redundant Communication Battery Charger".

Since we do not have experience with any redundant battery charging system, can you help us by sharing your experience in operating redundant battery charging system. If you will, please describe your system by answering the questions below. Do you have one of the following systems in service?

1) Do you have redundant battery charger configuration, say two (2) 48V DC rectifiers in parallel so that each is a backup rectifier for the other rectifier? The 48V DC load consists of communication equipment, SCADA, plus battery charging.

2) Do you use a transfer switch to switch the DC load to the alternate rectifier? The 48V DC load consists of communication equipment, SCADA, plus battery charging

Thank you.
Enoch Wu

 

Dear Mr Enoch Wu,

Two Battery chargers in parallel are very much feasible without a transfer switch. Let me explain. The outputs of both the chargers (in your case 48V DC) needs no changeover switch. The Chargers or Rectifiers, in the normal
condition, float charge a Battery Bank. The charger output voltages however, need to be kept approximately equal and slightly above that of the
Battery voltage during normal operation. This would ensure that the Battery does supply load in the normal circumstances.

Should one of the Rectifiers fail, the other Rectifier would continue to supply load. It is presumed that in this operation, both the rectifiers normally share the load in approximately equal proportions. The equal
sharing of the load by the two rectifiers, could be based on a voltage sensing and feedback circuit to the control card of the individual rectifier or the more stable method generally employed is to 'current sense' and feedback control.

There is absolutely no need of any transfer swich in the DC output side. However, you may, consider use of two contactors, one on each of the input AC side of the Rectifiers and in that event,
a) only one Rectifier is expected to supply the entire load at one point of time
b) the second Rectifier would take on the load in the event of the first Rectifier failing and a sensing circuit energising the contactor on the
input side of the second Rectifier. During the intervening period of soft start of the second Rectifier, the Battery would supply the load.
This is assuming that both the Rectifier inputs are from the same AC supply (three phase or single phase as the case may be).

For a typical SCADA application, keeping the objectivity of continuous assured DC supply, it would however be preferable to provide seperate
'sourced' AC supply at the input terminals of the Individual Recitifers. In this alternative, both the Rectifiers would be ON all the time, SHARE the
load all the time and in the event of one Rectifer failing or AC supply being interrupted or discontinued for a period of time, the second Rectifier would supply the entire load. This in Industrial applications of sensitive nature such as SCADA, is perhaps the most preferred solution.

There is one other advantage in the last mentioned solution. One of the two Rectifiers, can be made to supply load, while the other Rectifier or Charger can be used to Charge the battery (boost charge) after the Battery has
drained out due an extended outage of the Rectifiers. It is also possible, though not widely recommended, to make one Rectifier itself (in the event of the other rectifier being in continued inoperative condition), to supply
both the load and Charge the Battery simultaneously. This is done, by providing the supply to the load through 80 or 90 percent tapping on the Battery Bank. However, I must add that certain VRLA Battery manufacturers do not recommend the latter feature for fear of unequal charging of the Battery Bank.

Hope this is of some use to you in arriving at a conclusion
regards
venkatapathy sr
PS: the above statement is purely personal statement and does not in any
manner reflect the opinion of my Company