Author Topic: UL 1741 question  (Read 2382 times)

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johnnym

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UL 1741 question
« on: February 11, 2018, 01:01:21 PM »
Connecting to the grid seems like more trouble than it is worth. UL 1741 and/or IEEE 1547 approved inverters are very expensive. The purpose of UL 1741 and/or IEEE 1547 is "UL 1741 SA and Rule 21 enhance anti-island testing to ensure PV systems disconnect when required. In contrast, voltage and frequency ride through and ramp rate control testing ensure PV systems act in a predictable fashion as the grid experiences fluctuations to its operational thresholds." So protection in power outages and the such right?

If grid power goes down then why can't I use a dump load instead of having a UL approved inverter? Does this make any sense? It is all very confusing. This may have been discussed on this forum but an overload of good info exists here. A lot to learn here on this forum. Any feedback?

OperaHouse

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Re: UL 1741 question
« Reply #1 on: February 11, 2018, 03:11:40 PM »
I'm confused. What are you trying to dump. Some inverters will dump excessive power from grid tie into batteries. I'll be going to grid tie at my off grid camp this year.  The grid ties do take time to spool up, so I will be using a fast dump to heat water as the load changes.  Can't say much till I have actually done it.

dnix71

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Re: UL 1741 question
« Reply #2 on: February 11, 2018, 09:31:18 PM »
In most places you can pay extra for battery backup and use that to continue off grid during a power outage. Many installers do not recommend this because a transfer switch and genset is usually cheaper and much easier to service. Plus the cost of maintaining batteries has never come down like the cost of solar pv has come down.

There are only a few places in the world where the grid is so strained that distributed generation by solar grid tie is actually welcome. Put yourself in the shoes of the utility company and ask yourself if you really want private micro backfeed tied to your expensive grid. If any one of the thousands of house inverters fails to isolate itself bad thing can happen quickly

joestue

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Re: UL 1741 question
« Reply #3 on: February 11, 2018, 11:06:09 PM »
Put yourself in the shoes of the utility company and ask yourself if you really want private micro backfeed tied to your expensive grid. If any one of the thousands of house inverters fails to isolate itself bad thing can happen quickly

does not matter, nothing bad will happen. the grid is too much of a load for anyone or even 10% of the houses to support. of all my reading on the internet in the last 15 years i can only find one and only one unconfirmed public testimony of a utility worker who found an entire subdivision being backfed by a couple people that had generators... and guess what no one died.

one death is worth about 8 million dollars or something to the epa. the money spent to make everyone's grid tie inverters pass the ul1741 standards exceed this by orders of magnitude, and no one has ever died from someone's local power grid being backfeed by an inverter. and even if a couple lineman in the last 30 years have died from home owners who lost power and back fed their pole transformer (blown fuse at the pole) from a generator.. they died because they trusted the line was dead without testing it. and they knew the utility side was hot. parasitic induction is enough to make lethal voltage show up on a 7200 vac transformer secondary... rules are simple: if its not grounded don't touch it.

anyhow,
the reason they don't want to pay you any money for your solar power is simply because the actual cost of power is about a third of the residential rate. .. this is about how much they give you for it. its only after state and federal subsidies that my neighbor (seattle area) has paid off his solar panels in less than 5 years.

the more plausible reality ignoring the subsidies and politics is they just don't want to have to forecast the weather.
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johnnym

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Re: UL 1741 question
« Reply #4 on: February 11, 2018, 11:47:21 PM »
I would be using a small wind turbine and two 100 watt solar panels (That is what I want to do at least. It is possible to do this so why not?). The power would not exceed 1000 watts unless hurricane/gale force winds were present (Even then I would use the stop switch). The avg. amount of power would be between 100-600 watts with the wind turbine and around 150 for the two solar panels. May be saving 5% on the utility bill. Not even really worth it but I have the items and they are not being used at the moment, so why not save 5% on the bills? Does this make any sense?

joestue

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Re: UL 1741 question
« Reply #5 on: February 12, 2018, 01:24:20 AM »
I would be using a small wind turbine and two 100 watt solar panels (That is what I want to do at least. It is possible to do this so why not?). The power would not exceed 1000 watts unless hurricane/gale force winds were present (Even then I would use the stop switch). The avg. amount of power would be between 100-600 watts with the wind turbine and around 150 for the two solar panels. May be saving 5% on the utility bill. Not even really worth it but I have the items and they are not being used at the moment, so why not save 5% on the bills? Does this make any sense?
don't be mentally ill.


you would probably need several million dollars bond on file with the city for a mere... 'well' to be added to the municipal water supply.


want to offset your own demand? sure, go ahead. we should not be able to determine if you did or didn't...
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clockmanFRA

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Re: UL 1741 question
« Reply #6 on: February 12, 2018, 02:42:49 AM »
I have been messing about, with AC coupling now for the past few years.

"Messing about" is not really the word I would like to use, but getting hard empirical facts in this day and age is not so simple as there are allot of armchair orators.

Okay my observations.

The GTI starts off as a slave to the Grid and needs to see a stable HZ and a stable ac voltage within the GTI's parameters, and needs to see these for a pre set determined time.

I have a particular brand where according to the manufacturers the Anti Island can be turned off, however when AI is turned off the GTI still needs to see HZ and ac voltage with in its parameters. AI turned off and the frequency can rise and slow output with an HZ rise. However the rise in HZ can be far to much and domestic appliances can fail to work properly.

So the GTI needs to see HZ and ac voltage stability. These I have tested and gotten used to what is good and what is not so good.

Normally the main utility Grid is stable and you can back feed to it that's if your Electricity supplier will allow back feeding. If its a disc meter then it will spin backwards. If its a electronic meter then the back feed will not be allowed, a red light comes on, and the meter will register internally to your utilities electricity supplier.

So its a fine line to stop back feeding to switching to a dump load.

As yet I have not seen a GTI that will continue outputting when its reference HZ and ac voltage is removed.

 As far as I can see GTI's are just not designed that way as its far cheaper to have them use a reference voltage than a fully stand alone Inverter with its own stable internally produced reference.


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frackers

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Re: UL 1741 question
« Reply #7 on: February 12, 2018, 06:32:12 AM »
Lets say you have the grid connection, the sun is out and even with the washing machine going you are still pushing power to the grid.

Then a power cut - no grid, so all you have is the grid tie inverter taking as much power as it needs from the solar panels to keep the washing going. Note that the inverter won't use all that the panels are capable of producing - it will regulate to a reasonable voltage.

Then a cloud comes across the sky - the input to the invert drops, the inverter tries to compensate but it can't output what its not getting on its input so the volts to the washing machine drops. The motor stalls because the voltage is too low. Stalled motors can draw a lot of current  so it starts getting hot. Next thing you know, the sun comes out and there is smoke wafting out of the washing machine.

Low volts and stalled motors I have personal experience of. I lost a fridge-freezer due to a lost phase further up the road which dropped the phase voltage to my house - about 100v instead of 240v. Smelt terrible!!
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Rainwulf

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Re: UL 1741 question
« Reply #8 on: February 12, 2018, 06:46:59 AM »
If you have a GTI and the power cuts, the output will die immeditely. It wont keep running the washing machine.

No GTI will feed power into a dead circuit. That's what inverters do.

GTIs have two error checking systems. One is that to feed power into the grid, it has to lead the voltage and current of the mains. So it "locks" to the grid frequency, and then starts to feed in power slightly ahead of the sine wave. A few degrees ahead in phase, and it puts in a bit more voltage. If the grid is 241.1 it might try 241.3volts.   If the grid fails, and its now only the local house that is taking the load, the voltage will immediately rise as the nearly 0 ohm impedance of the grid is no long there to absorb the power. Even if by some miracle the local load is exactly the same as the power the GTI is putting out, the local load still wont have its own frequency reference, and that frequency will fluctuate, which leads to the next error checking system.

The other error checking system is that if the grid is exactly 50hz, it will try to feed in at 50.1 hz. It will try and speed up the grid. Since thats impossible as the grid is a MASSIVE load, the error checking stays low, and the hz figure that its beating the grid at stays at zero.

There is a very specific reason for doing this extra hz feed:
So, lets say the grid drops at a local feeder station, but you have a whole bunch of people on a street all with grid tie inverters. Each one sees the load of the other houses, and the other inverters maintaining the voltage, so as far as the GTI is concerned it still has a grid right?

Except that every single one of them tries to increase the frequency a bit. Since there is no reference anymore, all of them start to speed up. That's then the signal they use to shutdown. Its called "anti-islanding" and absolutely vital to prevent an isolated "island" of houses being kept alive, and stopping power line workers from being electrocuted by what they think is a dead circuit.


joestue

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Re: UL 1741 question
« Reply #9 on: February 12, 2018, 03:14:56 PM »
 I dont know that the inverters try to push leading power into the grid.. if the utility had control over that... that would be damn useful.

But in the hypothetical case of a florida subdivision self powered with solar, it also doesnt work because the grid tie inverters dont smoothly decrease the power pushed into the grid as the voltage rises. So its going to trip due to undervoltage as soon as someones heatpump turns on drawing 60 amps lra..(due to the higher impedance of the gti) or over voltage as soon as supply exceeds demand
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frackers

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Re: UL 1741 question
« Reply #10 on: February 12, 2018, 04:42:50 PM »
If you have a GTI and the power cuts, the output will die immeditely. It wont keep running the washing machine.


I should have made it clear that I was painting a scenario where the GTI didn't support AI.

It's not just about safety but also about the fact that without AI, a GTI will just not work as expected.
« Last Edit: February 12, 2018, 06:09:16 PM by JW »
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