Emergency Grid-Tie Usage

[russ]

Sure it is. The Radian, for example, does exactly this - supports a fairly large grid-tied PV array and has all the functionality of an off-grid inverter/battery system. But again, that's spendy.
Likewise, if he just wants to "switch over" his array to supporting a battery bank,

Wrong thread maybe? He doesn't want batteries - he just wants to use the grid tied system while the grid is down.

[bcroe]

Likewise, if he just wants to "switch over" his array to supporting a
battery bank, then the Xantrex 80-600 will allow him to charge a bank from a HV array,
and the Classic 250 will allow him to do that from a lower voltage (but still GTI capable)
array.
I think a lot of people think "switches, relays and wires have GOT to be cheaper than a
$600 charge controller!" but often that's because they haven't added up what good (600
volt rated) disconnects and weatherproof relays actually cost.

I think cdevidal showed a plan with no batteries, and changeover to parallel panels
was to be done by hand on a ladder. Any details of this 36VDC high current bus
system haven't been addressed. Bruce Roe

[billvon]

What you want to do is not sold on the market. Any idea why?

Sure it is. The Radian, for example, does exactly this - supports a fairly large grid-tied PV array and has all the functionality of an off-grid inverter/battery system. But again, that's spendy.
Likewise, if he just wants to "switch over" his array to supporting a battery bank, then the Xantrex 80-600 will allow him to charge a bank from a HV array, and the Classic 250 will allow him to do that from a lower voltage (but still GTI capable) array. Again, somewhat expensive.

What he wants to do, I think, is have similar functionality without the cost. Which of course there are problems with. I think a lot of people think "switches, relays and wires have GOT to be cheaper than a $600 charge controller!" but often that's because they haven't added up what good (600 volt rated) disconnects and weatherproof relays actually cost.

[russ]

Connectors and inverters? All of these components are sold on the market. It's just a matter of if I'd like to pay for a second inverter/charger/etc. Maybe, maybe not.

Alright thanks all, I've gotten what I need from this forum thread. If this doesn't help me hopefully it'll help someone else. Checking out.

Help someone else? How? Don't give yourself any credit for charging down a dead end street.

[cdevidal]

What you want to do is not sold on the market. Any idea why?

Connectors and inverters? All of these components are sold on the market. It's just a matter of if I'd like to pay for a second inverter/charger/etc. Maybe, maybe not.

Alright thanks all, I've gotten what I need from this forum thread. If this doesn't help me hopefully it'll help someone else. Checking out.

[cdevidal]

I am saying, you haven't covered any info on your loads. And you haven't shown
any detail of what equipment goes between the panels and the loads. Bruce Roe

Didn't think that was relevant? At this point I'm not sure what loads. I simply wanted to know if it is technically possible/safe, and from what people are saying, it is. Next I've got to decide if it's financially worth the investment in an off-grid inverter, charger, etc. that's going to be on standby 99% of the time. It might be worth doing for small loads.

[russ]

What you want to do is not sold on the market. Any idea why?

[bcroe]

I'm sorry but I don't understand your concern. I'm only discussing breaking the connection to a string or micro and re-connecting that to an appropriately-sized charger or inverter. (I don't have to use a DC appliance if people don't generally use that.) The goal here is to have both grid tie and off-grid circuits available (assuming the price is right) sharing only the panels. If you design for both from the beginning using standard practices, and disconnect/reconnect using standard safety practices is there any risk? I don't understand your concern. Please elaborate.

I am saying, you haven't covered any info on your loads. And you haven't shown
any detail of what equipment goes between the panels and the loads. Bruce Roe

[cdevidal]

I assume that you don't want to do this right because you don't want to spend the money?

Well I wouldn't do it if the price is too high, but I don't know that yet. As I said, I have a generator already and this would be plan B. I think what people are saying is there are no technical hurdles short of the usual safety considerations of disconnecting any panel (which everyone has to deal with anyway) and now it simply becomes a cost vs. value proposition.

[billvon]

I'm only discussing breaking the connection to a string or micro and re-connecting that to an appropriately-sized charger or inverter.

I assume that you don't want to do this right because you don't want to spend the money?

[cdevidal]

I would never try a blanket...

(..)

...and to ground, is for your safety.

Valuable information, thank you. Obviously before I tried it I'd educate myself better on the procedure

I wonder about your 36VDC plan. This is a pretty serious design problem
in all sorts of ways. All your loads will need need to be set up, to properly
operate on this. We do the same thing in phone offices (48V), using local
DC-DC converters wherever needed. An interlock system is used to prevent
any current flow during connect/disconnect.

Probably the same converters would work for you, but just working out your
load designs is non trivial. How will the system perform with clouds coming
by, or the sun gradually moving lower? Unless the load is quite small, it
would be a nightmare to control. Bruce Roe

I'm sorry but I don't understand your concern. I'm only discussing breaking the connection to a string or micro and re-connecting that to an appropriately-sized charger or inverter. (I don't have to use a DC appliance if people don't generally use that.) The goal here is to have both grid tie and off-grid circuits available (assuming the price is right) sharing only the panels. If you design for both from the beginning using standard practices, and disconnect/reconnect using standard safety practices is there any risk? I don't understand your concern. Please elaborate.

[bcroe]

Grin. So, I'd do what everyone does when disconnecting panels. I suppose that people either work at night or cover with a blanket.

I would never try a blanket; even light hitting the back side can produce
dangerous power. Sometime I do work in the dark with a head mounted
light, but its mostly to avoid power production downtime.

To add to the above, I stop current flow by opening the inverter AC breaker.
My inverters ground DC negative, so then I open BOTH lines with a DC
breaker as noted above. Since current is stopped, it won't be so hard
on that breaker.

Next I might run a test to make sure there are no grounds in the DC system.
Using a test light like this between ground and either DC polarity will
reveal any serious leakage. A digital meter can't be used, since it won't
draw enough current to drain harmless static charges.

PVTstLt.jpg

After that break the system into smaller, less dangerous sections. First
is open combiner breakers/fuses. Next could be unplug a panel near
the common polarity, negatives are tied together in my combiner. At
this point no string should show any power at the combiner.

Next I'd unplug at the center of each string. Continue to break things into
smaller groups. Using a power presence tester between terminals, and
to ground, is for your safety.

I wonder about your 36VDC plan. This is a pretty serious design problem
in all sorts of ways. All your loads will need need to be set up, to properly
operate on this. We do the same thing in phone offices (48V), using local
DC-DC converters wherever needed. An interlock system is used to prevent
any current flow during connect/disconnect.

Probably the same converters would work for you, but just working out your
load designs is non trivial. How will the system perform with clouds coming
by, or the sun gradually moving lower? Unless the load is quite small, it
would be a nightmare to control. Bruce Roe

[cdevidal]

One thing I remember are the plug inverters the guy was using. Illegal to use and unsafe

His panels were sitting on the ground and propped with a piece of wood.

The electric connection area was totally rube goldberg.

Grateful!

[cdevidal]

Again, you still have the problem of disconnecting under load.

Per the post just above yours, now I'm considering connectors and using a blanket.

We're not talking about something I'd need to have for a quick switchover; I already have a generator and gas for short-term outages. This is when the outage has stretched for weeks and it's time to move to plan B. Or if the generator doesn't work. Get out the ladder, cover the panels, pop the connector, re-wire to something different, done. Move to the next panel.

-Use connectors rated for outdoor usage. MC4 connectors (which many panels come with) are an example.

Ok, perfect.

-Break BOTH sides of the circuit. You can do this with a three phase safety switch. For example I use a three phase Square-D disconnect rated for 30 amps 600 volts DC. Get a disconnect like this and switch both positive and negative sides of the string. That will reduce your risk of electrocution.

Good call.

-Follow the usual rule of not paralleling more than two strings without a fuse.

Good info, thanks.

This will allow you to rewire for a lower voltage but again I'd be very careful assuming that you can just use an AC appliance with a DC source.

Well, that idea came from a what has been called a "loony land" link. The risks have been outlined above: "Not all loads can tolerate a varying DC supply", "If the thing has a transformer in it you'll blow the appliance fuse (best case) or smoke the transformer," etc.

So I've dropped that idea in favor of things that people typically connect to a single panel: a small battery charger, or an inverter, or whatever.

(There are plenty of Youtube videos of people trying to use AC switches to break a DC circuit. They are sometimes pretty exciting.)

Why not just get a SMA inverter with the secure power supply?

That's not out of the question. Though I do like the flexibility. If I wish for more than 1500W I could run all of the panels in series to a different inverter -- noting the precautions above -- assuming I wish to invest in another inverter and not just more gas, and assuming I wish to take the precautions with the electricity.

So it seems at this point we've discovered that this method has inherent risks but it's not entirely unworkable. At this point it becomes more of a cost vs. value proposition rather than will it work.

Thanks forum, you've been most helpful.

[russ]

Problems with the link?

One thing I remember are the plug inverters the guy was using. Illegal to use and unsafe

His panels were sitting on the ground and propped with a piece of wood.

The electric connection area was totally rube goldberg.

Many people pos all sorts of trash on the net trying to make themselves look important.