Grid-tied Photo Voltaic (PV) Solar

There is one additional reason for shutting down the PV Grid connect system when the grid goes down - safety.

That also applies if you have battery back up.

To have a home generation system that continues to supply power when the grid goes down, first you MUST disconnect your house from the grid because

If your PV system continued to deliver power when the grid went down:

1. it would be trying to power up the grid, not just your house!

2. any linesmen trying to correct a line fault would be at risk from the power being fed into the grid

Once your house is disconnected from the grid you could:

1. do as suggested above and have a battery bank, OR

2. connect a gen set which would provide the regulated, synchronising base power to enable your PV system to arc up and supplement it

This however is no trivial task or system.....

The batteries act as a reservoir for outgoing power and as a ballast for incoming power. I have installed quite a few grid-tied, battery back-up power systems (PV, Hydro, etc) and they work fine when the grid goes down. The inverters designed to be only grid-tied (SMA Sunny Boy, etc) are designed to power down with the grid. I imagine a inverter could be built to draw available PV power and supply to loads directly but they are not practical in the commercial sense.

Grid tied solar systems require current from the grid to synchronise their output to the grid, so no grid power, no output to the grid from the PV system. Remember grid tied systems draw power from the grid, not directly from the PVs, although in practice this may happen.

This is also a safety issue for people working on the broken grid system, they don't want power coming down the line/s from PV systems when they are repairing the fault/s!

I'm getting a Pv system installed in the next few months (before July 1). without any battery back-up. It will have an inverter though. The Pv system will supply household usage first, any "excess" is than sold into the power grid. All I can suggest - on thinking ( NO - no local brownouts !! ) is as the inverter IS connected to the grid (to regulate the current etc) - that maybe how the comment has arisen. Other posters explain it better.

As discussed earlier- it is possible to support your site first, and then feed the grid with excess IF the grid is operational. I believe that the referenced standards contain a means of "switching off" the grid if it goes down- similar to a transfer switch that sends generator power to local loads if the grid goes down and the generator starts.

As stated very well previously, the reason for the isolation is to protect line workers who "think" that the line they are servicing is dead- yes, they are SUPPOSED to test it, BUT- most of the electricians who get electrocuted have many years of experience and they get sloppy working with power because of that experience.

Even without a full battery back-up, you could use a small UPS system to create the reference voltage/frequency that your inverter will need for reference.

Hadn't thought about a UPS system for referencing, as we rarely have power outages in this area. Certainly something to consider in the very near future !

I have heard that this will work ...using a standard grid tied inverter to supply the reference voltage and frequency for Grid Interactive Inverters such as Enphase .. but you should choose an inverter that will generate as pure a sine wave as possible since the Grid Interactive Inverters require some pretty tight tolerances to the actual grid voltage and frequency ...

I believe that it is a legal requirement that when the grid goes down any grid connected inverter also goes down.

If you want to power your house during a power outage it MUST first be disconnected from the Grid.

I was looking at the manual for my Grid connected inverter yesterday and it illustrates multiple inverters in parallel connected to the grid at a single location - The Grid is such a low impedance load that when it goes down no conceivable home system is going to be able to hold it up enough for the outage not to be detected by the inverters.

To locally power the house would require disconnection also from a practical point of view, then bring a local generator of what ever form you choose, on line - this would then permit the panels and inverter to contribute also.

Again, before reconnection to the grid, the home system would need to be shut down or you run the very high risk (probability close to 1) of a partial antiphase connection which would be VERY bad news for everything in the house! - Huge surge currents......not pretty!

Just my two sense.

WAWAUS gave a good explanation of what would happen if a "grid-tied" PV array did not disconnect itself from the grid, hense the term "grid-tied". The reasoning and rules for this can be found under UL 1741

Typically an islanding condition may exist when the utility is suddenly disconnected and the remaining load on

the inverter is nearly balanced to its output power. [9] Tests in UL1741 and IEEE Std. 929-2000 add a tuned LC

(inductive and capacitive) circuit in parallel with the balanced resistive load. [1O] The total RLC circuit has a

quality factor (Q) of 2.5 that tends to hold the resonant frequency near 60 Hz. Inverters meeting the standard are

able to detect the islanding condition described above and disconnect within 10 cycles if the load is less than 50% or

greater than 150% of the output power of the inverter and the power factor is less than 0.95 (leading or lagging). If

the load is balanced within the 50'%.limits and the power factor is greater than 0.95 then the inverter must

disconnect within 2 seconds when the test circuit has a quality factor of 2.5 or less.

Fron Sandia SAND 2000-2437C

All grid tied inverters are required to "disconnect themselves from the utility, IF you would like to have some form of battery or generator back-up that is very much a solution, Most Outback and other off grid manufacturers (UL listed) have outputs to start a backup generator or another form of energy for the house. You could even go as far as have a Bi-modal (grid/off-grid) inverter controlling a cheaper, grid tied inverter.

There are ways.