Hi: It is possible for a grid tie inverter connected to the output of a battery based inverter to export energy through the battery based inverter to the AC source. It doesn't matter is that source is the grid, a generator, or any other AC power source.
Indeed, due to this common AC bus wiring structure inside the battery based inverter, there is no way for the battery based inverter to prevent this, other than disconnecting. Here's a line diagram of a typical battery based inverter with a grid tie inverter connected at its output. In the diagram you can see that, 1. All the AC sources and loads are on a common AC bus consisting of only copper wires and a couple of disconnects. . This structure of true of all of the grid, that is, there are multitude of AC sources and loads all on a common grid of copper wires and disconnects. 2. The only thing that the battery based inverter can do is, in charge mode, behave like a load as it draws power from the AC bus and charges the batteries or in invert mode adjust its AC output voltage so that it becomes a parallel AC source with the generator and is able to supply power to the AC bus concurrently with the generator. Here's where it gets interesting, 3. If the battery based inverter is in invert mode and a source such as a gird tie inverter raises the AC voltage on the AC bus, then power will flow in reverse through the transformer and the batteries will be charged using the grid tie inverter power as the source. This is AC coupled charging. 4. At the same time, if the grid tie inverter raises its output voltage above that of the generator, power will flow into the generator. The diesel motor in the generator will in this case be under a positive load, which is the same condition as a truck using engine brakes. 5. Unfortunately most generators are not built to operate as an engine brake, so bad things can happen to them. For this reason, the control algorithm in the battery based inverter will disconnect from the generator to protect it from damage. There is more details to this in regards how many residential loads are on, the quality of the generator output power waveform, methods to control the AC coupled charging, etc., but the above is the essence of how it works. JARMO _____________________________________________________________________________________ Jarmo Venalainen | Schneider Electric | Solar Business | CANADA | Training & Development Specialist - Senior Phone: +604-422-2528 | Fax: +604-420-1591 | Mobile: +604-505-0291 Email: jarmo.venalai...@schneider-electric.com | Site: www.SEsolar.com | Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 *** Please consider the environment before printing this e-mail From: "William Miller" <will...@millersolar.com> To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org>, Date: 09/17/2014 10:20 PM Subject: [RE-wrenches] Sunny Island sending power to the generator?? Sent by: "RE-wrenches" <re-wrenches-boun...@lists.re-wrenches.org> Colleagues: I was invited to troubleshoot a Sunny Island system. What I found is a Sunny Island mini-grid system behaving oddly. The generator, when started with the SI, would run for a few minutes and then shut down. Further investigation revealed that the SIs were indicating that the inverters were sending lots of power back to the generator, even with the Sunny Boy turned off. I don’t know if this was actually the case, nor can I grasp how this could happen. The system was programmed as a PV/Gen only system. Fortunately I reached an off-grid tech at SMA and we were able to resolve the issue. I am not sure what we did but I suspect that simply rebooting the Sunny Islands was the cure. I post this in case any of you experience this problem and could benefit from my experience. Also, I want to understand this better. Could it be true that the inverters could send power to the generator? I know the circuitry is there to sell to AC2, which is how this system would work as a true grid-interactive system, but how did it get triggered? Was it battery-selling to the generator? I welcome any comments. What I am starting to think is that the Sunny Island system, like any feature rich system, is complicated and has the potential to be less reliable than simpler systems. On the plus side I could also think of these systems as job security for the knowledgeable technician. Sincerely, William Lic 773985 millersolar.com 805-438-5600 ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. ______________________________________________________________________ _______________________________________________ List sponsored by Redwood Alliance List Address: RE-wrenches@lists.re-wrenches.org Change listserver email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out or update participant bios: www.members.re-wrenches.org
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