question

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Off Grid with Stand By generator

I have some simple questions that I have not been fully able to flush out in all my research and am hoping that someone will be able to give definitive, simple straightforward answers so I don’t make a costly mistake. (1) Of all the inverters what will be the best option for an off Grid Setup with a back up generator. I can see several options but I am not sure if I should be using a Quattro or and MultiPlus II. I want the generator to back stop the system in case the renewables are not enough to run the system and the batteries are getting low. I will be using a combination of Hydro, Wind and Solar.


Requirements

  1. Max current out put 300A (200 A house panel and 100 A Shed Panel)
  2. Output Voltage 240V to 120V
  3. Generator activation when batteries hit predetermined low level
  4. Battery Charging from Generator
  5. Battery Bank 48VDC

I have attached my rough design on what I plan to do. The battery bank will be built out of LifePo4 batteries creating a 48VDC system and storing around 9600 ah. The batteries will have an average normal discharge rate 320 A per hour with a peak over 1000a but will need to be under 10 mins.

  1. If the batteries are full and wind/hydro/solar are still producing power
    1. Does the MPPT prevent the extra power from over charging the batteries?
    2. If so what doe the MPPT do with the extra power? I known in some hydro system they have a burn off resister.
  2. Can multiple Autotransformers be placed in parallel? I have seen several references in post about needing multiples, but no one has explicitly said they can be used in parallel. I need to have a 200A circuit of 120V L1, L2 for the house.
  3. Is there any reasons why a Solar Charge Controller could not be used with a wind turbine if properly sized? They hydro turbine I plan to buy have been tested with the Victron MPPT(s) and they even recommend a model to be purchased based on what I buy from them. The wind turn bine has no suggestions from the manufacture.


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GeneratoroffgridAutotransformerHydro and Wind Power
diagram.png (237.3 KiB)
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1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

1) The MPPT only prevents overcharging of the batteries if the pack voltage exceeds the set limit. If - as usual- a cell exceeds its voltage, the pack voltage may not be over the limit. A BMS integration to the MPPT's is required to do this. The MPPT's cannot 'dump' the excess power, for the solar this is no problem, but for the Hydro, this may cause overspeed of the turbine.

2) Auto transformers can be used in parallel, but keep wire lengths and sizes identical.

3) The reason for not using a solar MPPT with a hydro generator is basically as the unit sweeps for max power, this can cause excessive input currents which could damage the unit. Also, as the mppt comes off load due to voltage / current limit, the turbine becomes under-loaded and may over speed. PV panels are a current limited source, motors (eg hydro-generator, alternator) are not.

It looks like you are using the three inverters in parallel, be careful to match cable lengths and sizes -particularly on the dc input circuit. Do not oversize the cables as the extra resistance helps to balance the load.

If you generator is auto-start type, then a 2 wire connection can be made to the relay in the master inverter, this can be programed to start the gen on low soc. Alternatively a BMS unit may gave a gen start output which will do the same.

Yes, the inverters will charge the batteries from the generator.

I have used 2 smaller Multiplus inverters in parallel for several years with relatively little trouble. It looks like you should add a GX control unit to your system, and a bms for the lithium battery. Make sure that the BMS and the GX can be integrated. This is essential to prolonging the life of lithium batteries as it limits over charging and over discharging of the battery.


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