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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 ?
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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