[RE-wrenches] Sunny Island sending power to the generator??

Thu Sep 18 10:36:47 PDT 2014

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.venalainen at 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" <william at millersolar.com>
To:
"RE-wrenches" <re-wrenches at 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-bounces at 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

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