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This question is just out of curiosity.

Let's say I have a mains-connected PV system installed over my roof with a phase-tracking inverter. Now main's is out for some extended time and I need to live with electricity, but without the mains the inverter wouldn't work. Can I bootstrap this mains inverter with:

  • A diesel engine-generator set?
  • A wimpy-powered offline UPS with PWM-based sine wave output?

My intention is to feed the output of the equipment into the mains input of the inverter to make it track on something and push out power, which in turn can be used to power the house.

Is this possible? How to implement this?

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    \$\begingroup\$ In general, a PV grid-tie inverter assumes it can dump all of the power generated (after losses) into its output. Normally, excess power is absorbed by the grid and excess draw is provided by the grid. If you use a UPS or small generator, you'll need to match your load very close to the current PV output. If something changes (your load goes below the output of the PV or above the output of both PV and generator), the inverter will fault and trip off. This is assuming you can get the generator output frequency close enough to satisfy the inverter. \$\endgroup\$
    – DoxyLover
    Jan 20 '15 at 19:36
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    \$\begingroup\$ Also, keep in mind that what you are proposing is likely illegal and, if you are not careful with killing the main breaker, you could kill a linesman trying to restore your mains. DON'T DO IT. \$\endgroup\$
    – DoxyLover
    Jan 20 '15 at 19:38
  • \$\begingroup\$ @DoxyLover I have no control over the main breaker as the grid company here controls them. And if I used a UPS can I make it coordinate with the PV by absorbing excessive energy PV produced with the batteries? \$\endgroup\$ Jan 20 '15 at 19:53
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    \$\begingroup\$ If you cannot disconnect from the mains, don't even try. If your outage covers additional residences, you'll be trying to power all of them and you'll likely trip off. If it's just you, you'll have a good chance of injuring or killing the electric company employee trying to restore your power! Give it up! \$\endgroup\$
    – DoxyLover
    Jan 20 '15 at 21:03
  • \$\begingroup\$ If your load exceeds what the grid-tie inverter can put out, there shouldn't be any big problem paralleling with a generator. But what happens when the load drops for some reason and the inverter tries to dump more power than the load wants? I would only do it if I knew I could absorb the full output (and disconnect from the grid somewhere along the line). To be honest, I wouldn't do it. I would just use a backup generator and a transfer switch. \$\endgroup\$
    – mkeith
    Jan 20 '15 at 22:17
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While what DL says in comments about lineman safety issues is technically correct, a formal study of real world conditions indicates the lineman is more likely to die from tripping over your cat than from electrocution. (They did not use a cat-metric in the study but that was the order of result.)

What DL says about legality is correct - a grid tie inverter is invariably required to have "anti-islanding" features and while creating an island that is isolated is potentially acceptable, creating an island that intentionally include an unknown number of neighbours and linemen is definitely not. Some administrations frown more heavily on people doing such things than others. I'd expect your administration to frown relatively heavily -but also that many people would do this.

IF you can achieve isolation then a motor generator with decent waveform may be enough. Means of anti-islanding and demands on the realness of the grid synced to vary quite widely.

It will be possible to delve within the internals of your inverter to make it islandable. You could die by two main means.

You can buy inverters that are made to provide islands at a modest percentage of full power - usually good quality and duly costly. SMA Sunny island is one such and I think they may use SMA Sunnyboy inverters as island slaves. Looking at how SMA do do what they do may be educational.

Pseudo sine wave inverters of reasonable capacity are much lower cost than "pure sinewave" and less again that grid tie units. You may be able to LC or just L filter the output of these to a level acceptable to many loads. When using a cheap motor-alternator set long agoI found that a 1:1 isolating transformer tidied things up nicely.

You MAY be able to use an AC motor as an electro-mechanical grid stiffener to 'fool' an anti-islanding system, - run up motor on whatever and then drop onto grid tie inverter output.

In the nearish future I wish to run anti-islanding inverters in true islanded modes and will be investigating means of doing this. I do not wish to power the neighbourhood or excite linemen.

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  • \$\begingroup\$ Or maybe I can hack my offline UPS to make it being able to take DC from the PV as a power source, give it an output rating boost by swap out the original power MOSFETs with more powerful IGBTs and replace the current shunt with a smaller one, and when the grid is out, unplug the mains completely and use the hacked UPS to draw power directly from the PV, charging the batteries in the day and drain them in the night... Just what if I find out that my power usage (maybe minus HVAC) can be fulfilled completely by the PV power... Go off grid maybe? \$\endgroup\$ Jan 21 '15 at 16:05
  • \$\begingroup\$ You sound keen:-). However, there is also the issue of the transformer in the LV to HV DC inverter which is liable to be run at near full rating already. If your battery DC voltage was at about mains peak potential then you could convert it directly, but, I imagine that that is not the case. \$\endgroup\$
    – Russell McMahon
    Jan 21 '15 at 16:46
  • \$\begingroup\$ Well it seem to me that they used switching elements directly working on 12V battery voltage and used a mains-rated transformer to push out mains. They do not have a separate inductor after the (presumably) H-bridge. \$\endgroup\$ Jan 21 '15 at 16:58
  • \$\begingroup\$ @MaxthonChan The "mains rated transformer will have a voltage and amp-turns rating so a current rating. This is usually limited by core saturation. Double power usually needs ABOUT double volume core. \$\endgroup\$
    – Russell McMahon
    Jan 21 '15 at 17:17

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