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#130225 - 02/22/06 08:11 PM DC voltage dip during outage
HighPotter Offline
Member

Registered: 08/29/04
Posts: 41
Loc: So. Cal
Recently, our facility had an event, where the station power dipped for approx. 50 cycles. At the very same time, with the battery chargers feeling this voltage dip, our 130v dc battery bank voltage dipped to 78 volts for 10 cycles, then recovered fully.At the time, the batteries had a 15 amp load. The batteries have just had there regular maintenance and testing, and everything seemed fine. They were not however load tested, which is not part of our companies SOP.

Would a 10 cycle dip be normal? These are simple flooded lead acid cells

HP

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#130226 - 02/23/06 09:24 AM Re: DC voltage dip during outage
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
HP - I'm a little confused by your use of the term 'dip' for both the AC event and the DC response. Is it the case you experienced an outright outage on the AC for 50 cycles, which then produced a 52V sag on the DC for a 10 cycle duration?

On the face of it, I'd guess that the magnitude of the DC sag (52V) is of greater concern than the duration (10 cycles). I think I'd arrange a test discharge to determine current battery capacity.

Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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#130227 - 02/23/06 03:00 PM Re: DC voltage dip during outage
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
HP,
I would bet that you have at least one dead cell in your battery string. They will be worthless under load but act as a decent filter capacitor for the phase control amplifier to look at a smooth voltage. If your input dips, it might take a few cycles for the firing angle of the SCRs to skew to make up for it. That's my guess from the info that you have provided.
Most of the newer chargers run SCRs these days.
Joe

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#130228 - 02/23/06 03:27 PM Re: DC voltage dip during outage
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Defineitely a problem with the battery set. The idea is to have a stiff DC source to trip AC circuit breakers during faults.

Fortunately, a monitor is in place to warn of a potentially most seroious problem.

Get that battery set tested right away. The repercussions on disregarding the warning you found could be very expensive.


[This message has been edited by Bjarney (edited 02-23-2006).]

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#130229 - 02/23/06 08:10 PM Re: DC voltage dip during outage
HighPotter Offline
Member

Registered: 08/29/04
Posts: 41
Loc: So. Cal
Radar,

No. It wasn't an "outright outage" on the ac side. The station felt an ac dip for 50 cycles. There was a large phase to phase and phase to ground fault on a few 7kv feeders out in the field, and station power "felt" this.

The suspect string of batteries are scheduled to be load tested asap. My guess still is that they are fine. I suspect the charger, and the was it handles voltage flucuations on the input.

Thanks for everyones opinions. I'll let you know how things turn out. I have been wrong many times before.

HP

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#130230 - 02/25/06 08:46 PM Re: DC voltage dip during outage
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
HP:The suspect string of batteries are scheduled to be load tested asap. My guess still is that they are fine. I suspect the charger, and the was it handles voltage flucuations on the input.

HP, I don't understand why you think the battery string isn't the problem. Our chargers are all fed by 208 single phase and have an AC input and DC output circuit breaker. We use a dual knife leading to 60 cells. We use much larger conductors to feed a DC distribution panel. The specified float range is 2.21-2.25 volts per cell. I set our chargers to 2.23 volts/cell or 133.8 volts total. We use 140 volts for an equalizing charge. Is this something similar to your system?

It would be almost impossible for the problem you described to be anything but the batteries. If you had a major fault across your battery load, you would likely trip a breaker on the DC distribution side. If you had a dead short in the output of your charger, you would almost certainly trip the output breaker of your charger. A typical problem in a charger will just reduce its output to zero, or overdrive it, or perhaps cause its output voltage to drift over time( the hardest ones to fix.) But none of these problems will be able to load a healthy string down to 70 some volts.

Our substation electricians record cell voltage and specific gravity once a month. I would bet that you have at least one cell that reads higher than the others under charging conditions but will let you down big-time if needed to feed a load. When I have tested our batteries, by turning off the AC breaker, I tend to notice the battery voltage increase over the first 15 to 20 minutes. It's as though they are waking up and getting used to flowing the 6-10 amps of normal substation load. (no controls) Then they start a slow discharge process. We don't use deep discharge testing of batteries due to concerns over shortening their lives.

I didn't realize how little I knew about batteries until I had to commission a charger and batteries in a new substation. The batteries were shipped dry and the electrolyte was added on site. I was shocked when I learned that I should bring the string up to 156 volts slowly by current limiting to 8 amps, and then leave it there for 3 days solid. I had never heard of using more than about 140 volts for an equalize, but it was in the C&D battery book. I followed the instructions and ended up with a perfect string, all cells within .01 volt and stark light/dark plate structure. We had another location where a substation had a new set of dry batteries. I advised the substation foreman to call me as soon as they activated them but he forgot. They were at normal float voltage for at least 2 weeks before I heard about it. I tried to undo the damage done by repeating the procedure described above but it was too late. The cells looked terrible and the voltages were all over the place. We will probably get less than optimal life out of them.

If you have never had a chance to cozy up to your battery manual, you might be surprised by what you'll find in there. The voltage that you say you float yours at sounds a tad low to me if you run a 60 cell string. Too low a float voltage will shorten useful life. Finally, when you replace your defective cell (s), it would be wise to prep the new cell(s) with a single cell charger or suitable supply.
Good luck with it!
Joe

[This message has been edited for typos.

[This message has been edited by JoeTestingEngr (edited 02-25-2006).]

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#130231 - 02/25/06 09:03 PM Re: DC voltage dip during outage
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
Joe - Batteries are complex things indeed. Frankly I've forgotten most of what I once knew, but I remember when I was in the Navy we used to to a test discharge of the submarine (SSN) main battery about once a year or so. We could remotely read the voltage of each cell, and we would do so continually after a certain point to make sure we did not get too close to damaging a cell by excessive discharge. The discharge itself was probably not the best for longer battery life, but under the circumstances, we needed to know the battery capacity, where we were in the life of the battery.

Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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#130232 - 02/26/06 04:09 AM Re: DC voltage dip during outage
Dnkldorf Offline
Member

Registered: 12/12/04
Posts: 1091
Loc: nowhere usa
Radar, and everyone....

You can damage a cell in a battery by excessive discharge?

Can you explain this a little more?


Dnk....

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#130233 - 02/26/06 08:46 AM Re: DC voltage dip during outage
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
Dnk - As I recall, we had to take care that we did not overly discharge a cell by continuing to run discharge current thru it. Think of it this way, when the weakest cell becomes completely discharged, there is still discharge current flowing thru it forced by the rest of the battery, and it will reverse it's polarity. The cell is essentially shot at that point.

Again as I recall, as a discharge continues, the rate of change of cell voltage increases. Since we had 126 cells and one physical meter with a cell selector switch, towards the end, cell voltages are changing fast and it's impossible to monitor all of them, so we'd pick the 5 or so lowest reading cells and just monitor those continually. When the lowest cell's voltage diminished to a certain (low but safe) point, we'd end the discharge.

There was ample technical description about how this happens in the Naval Ships Technical Manual, but of course I don't have one of those handy anymore. I cannot remember the specifics about why this happens, but submarine main batteries are big, expensive and no fun to change, so no one wanted to mess one up.

Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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#130234 - 02/27/06 03:02 AM Re: DC voltage dip during outage
jfwayer Offline
Member

Registered: 05/15/05
Posts: 30
Loc: Fairmont, WV, USA
A submarine technical manual is on the web. The following URL is for the battery chapter:
http://www.maritime.org/fleetsub/elect/chap5.htm#5A

The electrical system in a sub is facinating!

(Non trip free breakers so you can hold them on for instance)
_________________________
JFW

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