# Question about inverter-charger and AC coupled offgrid system

Registered Users Posts: 1
Hi everyone, I am new in the forum and I wanna ask you the following:
If we have a system like this:

how does the inverter-charger (in this example a Sunny Island) knows how much power he needs to absorb or feed? As it is bidirectional and it´s not connected to the rest of components...I read about the active power/frecuency and reactive power/voltage control, ( http://www.microgrids.eu/micro2000/presentations/4.pdf ; page 2 ) but I suppose this only apply with parallel voltage sources like synchronous machines, or another Sunny Island. In this case, Sunny boy and Windy boy work as current sources, right?

Is there any inverter-charger in the market suitable to make an installation of around 50 kW in single phase? Sunny Island is limited to 18 or 24 kW, don´t remember, then it goes to three phase cluster system.

Thank you

As I understand, the Sunny Island is the "master" of your micro grid. It sets the AC line voltage and frequency. The SI "tells" the other Sunny/Windy boys how much current (power) is needed by varying the line frequency (50 Hz+/- 1.0 Hz or so--guessing at exact value). When the batteries need charging current, the line frequency is 50 Hz (full power needed). As the batteries fill (and/or AC loads decrease), the SI varies the frequency by +/- 0.X Hz... The farther from 50 Hz, the less power is needed from the Sunny units (I believe the SI varies equal amounts above and below 50Hz--It keeps your clocks accurate).

You are correct the Sunny Units behave like "current sources" (follow AC line voltage from SI "master"). The backup genset (and/or Utility Power) which are "voltage sources" -- These have to be connected to an AC2 input on the SI (or other AC Battery Chargers)--Of course, you cannot connect a standard "voltage source" (AC Off Grid Inverter)--This will let the "magic smoke" out of the inverters+generators (the voltage sources "fight each other"--Each wants to set the line voltage and frequency).

Regarding larger off grid mixed power systems... If you do not have one (or you do not have the experience), you should probably find a consulting power engineer with lots of experience with your specific system/setup (and check references). Large system can have lots of issues that are not obvious (large AC power systems can do some remarkably insane power flow issues).

As an FYI--Here is one person that spent lots of time and money on one such system:

http://forum.solar-electric.com/discussion/17986/is-the-system-set-up-right

Good luck,
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 6,354 ✭✭✭✭✭
So are you offgrid or what is it?
I always find these big systems start out with alot of bad advice. (not you Bill, you are the Man!) It is usually much better to know alot more than the people who install it and let them do the work with your supervision, even the engineer if you have one. It does involve work on your part. To answer one of your questions.

"From a single Conext XW+ unit to clusters up to 102 kW, the Conext XW+ is a scalable system that allows for the integration of solar capacity as required. Adaptable and scalable, the Schneider ElectricTM Conext XW+ system is the one solution for grid-interactive and off-grid, residential and commercial, solar and backup power applications."

"we go where power lines don't" Sierra Nevada mountain area
http://members.sti.net/offgridsolar/
E-mail [email protected]