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Thread: Battery sizing for MV breakers

  1. #1
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    Aug 2006
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    Battery sizing for MV breakers

    Hello everone,

    I'm working on a project that will have a multiple MV switchgear, we are using a vacume breakers that will required control power. I have some information (from tech. spec sheet) regarding the loads that needs to have power from a 125V DC circuit (battery);

    Spring Charge Motor : Run Amps = 4A Time seconds = 7s
    UV trip mA Maximum = 80
    Voltage Range : close =100-140 Trip = 70-140
    Indicating Light Amp = .02A

    My question is, how I can do the math to size the required battery for this application. This is all new to me, any hit will be really appreciated. Thanks in advace

  2. #2
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    Feb 2003
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    You shouldn't have to do the sizing. The switchgear manufacturers sell the batteries as a package with the switchgear. They have already figured it out.

  3. #3
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    Quote Originally Posted by Nick
    You shouldn't have to do the sizing. The switchgear manufacturers sell the batteries as a package with the switchgear. They have already figured it out.

    Actually there is six Mv switchgears and I'm trying to have one centerlized battery to provide a back up power for all of them.

  4. #4
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    faresos,
    And don't forget a remote alarm for the battery charger. I worked on the replacement of a 5kV motor starter that blew up because a mouse got across the phases, but the batteries that supplied the trip voltage were dead. It took out two of the 34.5 kV primary fuses on the utility line that fed the plant substation. Had the batteries been able to supply the required current the feeder breaker that fed the 5 kV starter would have opened limiting the damage and keeping the rest of the plant online.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #5
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    I believe IEEE 450 I'll check in the morning and if I can find a link I will forward.
    Brian John
    Leesburg, VA

  6. #6
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    Do the breaker compartments have capacitor-trips in them? What are you trying to accomplish with the batteries? (just trip the breaker, operate the aux. equipment for an extended time, or...) Is the charger on a circuit supplied by utility only, or is there generator power also? Usually, the total wattage is low, a small battery pack goes a long way. Keep in mind that a typical trip coil on a MV breaker takes about 6 amps, but for a very short time.

    If you're designing this system, be sure the distribution equipment is rated for 125DC, most of it isn't. Most of the 125DC I've seen is isolated, not grounded in any way, and most battery chargers have ground fault alarms on them. If you're using circuit breakers, most of the 480AC ones are rated for 125DC as well, the 240AC ones are not. Most of the breaker panels I've seen are single phase, with 2 pole breakers, one 'phase' for + the other -.

    It's normally a good practice to place the batteries in a separate room, unless they're AGM type. It's amazing how far a small amount of gas can go. Also, if the battery room is kept at about 75F, the batteries will operate at near capacity, and last longer.

  7. #7
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    Batteries should be sized in accordance with IEEE 485, for lead acid. Ni-Cad should be in accordance with IEEE 1115/1115A.

    IEEE 484 also has info for design and installation practices.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  8. #8
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    That's what I get for working from this aging memory. You can download the specs for a one time price or buy a yearly subscription.

    http://www.ieee.org/web/standards/home/index.html
    Brian John
    Leesburg, VA

  9. #9
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    Redmond, WA
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    Switchgear Battery Sizing

    Look at manufacturers’ literature. Powell Electric and GE both used to have some good guides on switchgear battery sizing.

    Lacking that, develop a spreadsheet of continuous loads and momentary loads. Continuous loads include breaker status indicating lights, auxiliary relays with energized coils, power supplies for protective relays, meters, transducers, control PLC's, emergency lighting and any other load that may be connected to the battery and running continuously. Momentary loads are the breaker trips coils, lockout relays, breaker spring charging motors, auxiliary relays and other devices that will operate in less than a minute.

    Set up a typical time line for your system. Start with a power outage, assuming some breakers trip and recharge their closing springs in the first minute. Run the continuous loads for the duration of the outage, then show the breaker closing currents at the last minute of the outage, along with a main breaker tripping sequence.

    Provide the total amps versus time profile to the battery supplier and they will provide the recommended battery size based on the IEEE standards and calculations.

    Remember that the battery has to have enough power left after an 8 hour (or 48 hour) outage to power the protective relays, close the breakers and energize the trip coils if there is a problem.
    Bob Wilson

  10. #10
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    Quote Originally Posted by faresos
    Spring Charge Motor : Run Amps = 4A Time seconds = 7s
    UV trip mA Maximum = 80
    Voltage Range : close =100-140 Trip = 70-140
    Indicating Light Amp = .02A
    Do not mean to be a wet blanket here, :grin: but after working with battery plants for 30 years, you do not have enough load here to amount to a hill of beans. Even if you feed all four switches you don't even have a bowl of beans in terms of load for a given period of time.

    For example here is a very simple formula to get in in the ball park:

    T=AH/L

    Where:

    T= Time in hours
    AH = Amp-Hours of battery rating
    L = Load current (amps)

    Looking at your load comes out to less .1 amps, and that is stretching it. If you were to use 80 AA batteries in series with a 2.3 amp-hour rating would give you 24 hours or more of discharge time for 1 switch or about 6 or more hours for all 4 switches.

    The real key to what you are looking for is the max discharge rate a battery can supply over a given amount of time. In your case the maximum discharge you could ever experience is by some slim chance all 4 switches operated at the exact same time to give you 28 amps (4*7). Even the smallest industrial VRLA battery can more than meet that demand, so find the smallest one you can fit into an enclosure.

    Smallest VRLA I know of are rated at 25 AH with a 6X discharge rate meaning 150-amps delivered continiously for 10-minutes. These are 12.5 volt cells for small fire/securitity systems and in your application would work without a rectifier for more than 1 week.
    Last edited by dereckbc; 09-24-07 at 10:38 PM.

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