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Thread: Inverter sizing mystery

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    Inverter sizing mystery

    I have hit a block on figuring out what I see as a problem.

    We did a logging on a set of escalators to install back up power to last for the 2 ,5 hrs. of load shedding
    We logged 3 phases being even at +/-38amps with a PF of +/-0.7 and it was pretty constant as can be expected. Initially on installing the logger we double checked with a tong tester and the readings matched.
    Did the calculation and got +/-26Kva which is around 18.5Kw which again matched logger data - So we double check , voltage is 400v , no current on the neutral - voltage between neutral and earth is perfect as transformer is 20m away.
    There are spikes of 70 amps as the system is star delta ( there are reasons that VSD are not installed, which would be an obvious cost saving )

    My mind tells me that I need to size according to actual amps drawn and KVA so I estimate an inverter at 50KW to cover any spikes and allow a decent charge time in between load shed cycles , and a battery bank of 70Kwh
    I check with suppliers and they are nervous as my logger has recorded at 1sec intervals , they request we install a 1msec interval as they have bumped heads previously where the spikes are higher and longer then initially thought but agree that maybe the above system will be OK
    I sit with the client and run through the numbers where after they almost fell of the chair as we looking at around 800K for the system.

    They show me another price that is using 2 x 12Kw inverters in parallel with 40Kwh of batteries

    My reaction was that it will not work and if it does it will not last the 2,5Hrs of load shedding

    The 2 x 12Kw system gets installed - After install I am told everything works perfectly
    I have limited access to the system and check the inverter readings - On grid it shows current drawn at 25amps on one inverter and 14 Amps on the other which adds to my original +/-38Amps - I would be concerned on the imbalance but more concerning was that the inverter showed and accumulated 6 Kw load which makes no sense

    We run a grid failure test - The output of the 2 inverters shows current draw at +/- 32 Amps 380v with a multi meter , no current on neutral and the inverter shows an accumulate 6 kw load - The batteries deliver 6Kw of energy and after 2 hours show a decrease of 40% which matches the +/- 6Kw of load draw which equates to +/-16Kwh of battery

    What makes no sense to me is how the load still appears to be +/- 21Kva 14Kw and the actual draw from batteries and inverter show +/-6Kw
    Used 2 different makes of multi meters
    The slight discrepancy in multi meter readings and calc being offgrid 21Kva to 26Kva I would put to voltage and PF but to try explain the 6kw inverter draw is my struggle

    Can anybody think of a plausible reason ?

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    I understand that the communication between inverters to have them running a load in parallel is to synchronise the phases... and not much else.

    So is the 6kw draw just for that inverter? i.e You need to sum the value of each inverter to arrive at the total load.

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    Quote Originally Posted by Dave A View Post
    I understand that the communication between inverters to have them running a load in parallel is to synchronise the phases... and not much else.

    So is the 6kw draw just for that inverter? i.e You need to sum the value of each inverter to arrive at the total load.
    each inverter is showing 3 KW - the figures above I have accumulated already

    Off grid , on battery supply power , each inverter shows 16amp output on load with multi meter but only draws 3 Kw from battery and when I measure the current draw on each battery with a multi meter it shows 60 amps DC at 50 v = 3Kw

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    Site Caretaker Dave A's Avatar
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    The capacitors in the inverters are doing some serious power factor correcting?
    Last edited by Dave A; 10-Jul-24 at 09:38 AM. Reason: correction - inverter plural

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    Have attached screen off inverter showing readings along with DC tong test and AC tong test on load

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    Quote Originally Posted by Dave A View Post
    The capacitors in the inverters are doing some serious power factor correcting?
    If that was the case I would expect PF to be reading 1 and the calc would then be P = 400*1.73*1*16 = 11Kw per inverter and I would be back to +/- 22kw total load

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    Are the batteries common to both inverters or has each inverter got it's own set of batteries?

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    Quote Originally Posted by Derlyn View Post
    Are the batteries common to both inverters or has each inverter got it's own set of batteries?
    Think they could each have there own - separate fuses etc to each inverter - I took readings off battery cable going into the inverter

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    Quote Originally Posted by GCE View Post
    If that was the case I would expect PF to be reading 1 and the calc would then be P = 400*1.73*1*16 = 11Kw per inverter and I would be back to +/- 22kw total load
    Your 16 amp reading seems to be a sum of the current on all three phases.
    You multiply by 1.73 when you are using the current for one of the 3 phases ... (and this assumes a balanced load).

    Edit: Rather use the average current per phase for the calculation.
    And perhaps 380 rather than 400 as they are running at 220V per phase.
    (A 12 kW 3 phase inverter is actually 3 x 4kW single phase inverters).

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    Quote Originally Posted by Dave A View Post
    Your 16 amp reading seems to be a sum of the current on all three phases.
    You multiply by 1.73 when you are using the current for one of the 3 phases ... (and this assumes a balanced load).

    Edit: Rather use the average current per phase for the calculation.
    And perhaps 380 rather than 400 as they are running at 220V per phase.
    (A 12 kW 3 phase inverter is actually 3 x 4kW single phase inverters).
    The inverter readings added seem to show 16 amp total yet the multi meter reading on 1 phase is 16amp and it is balanced - all 3 phases show a 16amp draw/phase on each inverter making a total draw on the DB at 32 amp per phase after the parallel takes place

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