- Rep Power
ABB with Selectronic Off Grid Solution
I am putting this out there as information for others wishing to have a similar solution.
As a background :-
When a Hybrid system loses the grid, the PV inverter power has to be absorbed by something.
This usually is done utilizing some form of load to dissipate the excess energy, with some form of control also used to supply the house load as it varies.
The problem with this is that the energy is lost as heat when not used, even dumping into a hot water system, eventually the energy is lost when the water is hot enough.
Selectronics when coupling to ABB inverters uses their own system (Selectronic Certified) inverters, that provide managed AC coupling, what this does, is to reduce the PV generation when the energy is in excess of the requirements.
When using Generic AC coupling, the Selectronics causes the inverters to turn off utilizing frequency ramping.
This stops the excess energy overcharging the batteries, the problem with this is that the inverter will restart after one minute and the cycle repeats. Not very good for the inverter.
So its a good idea to purchase the managed AC coupling inverters from Selectronics from the start, it overcomes the above problem.
However if like me you already have the non approved inverters and added a Selectronic battery system you will have to put up with the things going on and off every minute when the grid is off, or spend more money replacing the perfectly good inverter.
I designed a management controller to do the same that Selectronic does, utilizing an ABB supplied component that controls the power output of the inverter, called a PVI PMU
This device is digitally controlled and can vary the PV power from 100% down to 0% in 11 steps, it uses the RS485 bus to communicate with the inverter.
The system I have developed, monitors battery voltage, further this voltage is temperature compensated to track the Selectronic charger that increases or decreases the charge voltage due to battery temperature variation.
Under normal conditions the battery voltage is maintained at the charge cycle as determined by the Selectronic charger.
The device monitor if the maximum voltage for the battery is exceeded, this than causes the PMU to start its control of the power output of the inverter reducing power to the allowed float voltage.
A balance is achieved when the PV will only be producing enough power for the house load and what ever the battery is requiring.
Should the house load increase, the battery voltage will drop below the Float Voltage, this is overcome by the PMU raising the PV output back to the float voltage balance point...
This float voltage has a capture window (User settable +/- voltage), this is how the fine control is achieved.
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