Two string Tee Connectors?

[jaggedben]

I'm not convinced that with lower labor costs for installation and longer product lives that string inverters are cost-competitive. Perhaps the up-front costs are lower for string inverters, but not the long-term costs.

YMMV. Still, as long as micro prices per watt remain about twice that of string inverters, it's going to remain a typical consideration. And often the up front cost is the decision maker, especially if the cost of money is high.

I believe the Y's are to be installed at each module, not at the very end of the string. If they are at each module, the performance of each module pair is controlled by the performance of the worst module in the pair. If they are at the end of the string, then the performance is the same as if they are going into a combiner box.

It's hard for me to understand how the overall system architechture would work if there were to be Y's at each module. They could not be put in series after that. The OP's numbers (and title of this thread) make it pretty clear that he was thinking of using them to combine strings.

 

[ggunn]

It's hard for me to understand how the overall system architechture would work if there were to be Y's at each module. They could not be put in series after that. The OP's numbers (and title of this thread) make it pretty clear that he was thinking of using them to combine strings.

The only way I could envision it is to have two Y's per pair of modules (one on the +'s and one on the -'s) and build strings by putting these pairs of paralleled modules in series, as if they were single modules with the same voltage but double the amperage. I don't see what that would accomplish, though, other than making for a wiring nightmare.

 

[tallgirl]

YMMV. Still, as long as micro prices per watt remain about twice that of string inverters, it's going to remain a typical consideration. And often the up front cost is the decision maker, especially if the cost of money is high.

I've installed large SMA sites and I'd rather not have to ever again, for systems below 30kW. I know what labor costs, and for the 7 SB6000's that were involved, plus the $15K Draker DAQ gear that was added on top to monitor what SMA doesn't monitor, I don't believe the installed cost would have been nearly as high had that particular system been done with factory-assembled AC PV.

I get that customers are often concerned more with up-front cost than TCO, I just happen to believe that falling materials costs and rising labor costs means that AC PV is going to be the winner. It's sort of inevitable. I missed the whole "What do you mean people will buy computers to play games?" trend in the early 80's. I'm not going to miss this one!

It's hard for me to understand how the overall system architechture would work if there were to be Y's at each module. They could not be put in series after that. The OP's numbers (and title of this thread) make it pretty clear that he was thinking of using them to combine strings.

That was explained just now by ggunn, but as I think we've all stated, no one any of us know has ever seen them in use. When I first read the literature, one pair per pair of modules is how the diagram was drawn.

 

[ggunn]

I've installed large SMA sites and I'd rather not have to ever again, for systems below 30kW. I know what labor costs, and for the 7 SB6000's that were involved, plus the $15K Draker DAQ gear that was added on top to monitor what SMA doesn't monitor, I don't believe the installed cost would have been nearly as high had that particular system been done with factory-assembled AC PV.

I get that customers are often concerned more with up-front cost than TCO, I just happen to believe that falling materials costs and rising labor costs means that AC PV is going to be the winner. It's sort of inevitable. I missed the whole "What do you mean people will buy computers to play games?" trend in the early 80's. I'm not going to miss this one!



That was explained just now by ggunn, but as I think we've all stated, no one any of us know has ever seen them in use. When I first read the literature, one pair per pair of modules is how the diagram was drawn.

IMO, those connectors should never be used when there are more than two strings in the system. That's what they are for; a two string system doesn't need fuses on the DC side.

 

[Zee]

MISMATCH
I have been exclusively using string inverters for years. On all systems I have gotten 0-15% better output than my estimates, whenever i followed up and checked. 100 resi systems.
Yes, panel level mismatch is real.
The degree to which it is relevant is - understandably - emphasized by micro mnfctrs.
However, I have found panel mfctrs:
1. now often individually test and sort panels by Isc
2. decrease their tolerance
3. and I have found very little variance in the field when i do measure amps, which i quit doing years ago.

Now, I am installing micros and (soon) AC panels full-bore.
But NOT because of panel level mismatch. The (free!) monitoring and 25 year warranty is simply unmatched. Damn, those old (several) thousand dollar monitoring systems were expensive, and cumbersome and serious labor! As TALLGIRL said.

 

[Zee]

DC combining connectors (Y's)
Who uses them and why? Hopefully, as GGUNN and Jben stated: two strings TOTAL.
No different than running 2 strings to a resi. inverter.

I'll admit right off the bat to having just used them. First time ever.
Unusual design not of my choosing.
Needed to add a string to another installer's existing system. Existing wiring down off roof was fortunately over-sized. So, i could parallel the strings on roof.

Not a problem per CODE or otherwise.

That is because, two strings in parallel will not be additive in amps, if ground fault occurs on either string!
If string #1 is shorted, then whatever current rushes in from 2nd non-shorted string will be flowing opposite the first strings current.
(The other scenario is if short is in inverter or elsewhere on combined output channel wiring. That conductor, as always, should have been sized to handle the double amps. But amps do not increase on modules, in this case.)
This dynamic changes with 3 or more circuits.
Hence no 3 or 4 string connectors. Only 2.

 

[SolarDude]

I have to say this first. I work for a solar installing company. And I have used 100's of these Y connectors over the years, but only for two evenly sizes strings of the same modules. I have never heard or seen a problem for any of Y conectors on the systems installed. And we monitor every system.

 

[ggunn]

I have to say this first. I work for a solar installing company. And I have used 100's of these Y connectors over the years, but only for two evenly sizes strings of the same modules. I have never heard or seen a problem for any of Y conectors on the systems installed. And we monitor every system.

As it should be, as long as there are only two strings on the system. The idea I was counseling against was that of reducing 4 or more strings to half that number by using the Y connectors and fusing the double strings at twice the single string rating. For a four string system that would be like using no fuses at all.

 

[esquared]

As it should be, as long as there are only two strings on the system. The idea I was counseling against was that of reducing 4 or more strings to half that number by using the Y connectors and fusing the double strings at twice the single string rating. For a four string system that would be like using no fuses at all.

I've been following this thread with interest because a client is insisting we utilize these y-connectors for a megawatt+ install.
I have given my objection because of the issues previously pointed out:
1. Module indicates maximum series fuse rating of 15A - (to me, this should end the discussion)
2. Other properly operating paralleled strings in the combiner, (i.e., not one of the strings in the faulted pair), can potentially contribute fault current that is sufficient to damage the module, but not blow the fuse.

To a certain degree I can agree with point 2. However in my scenario this is highly unlikely. If there were a fault, the local current (i.e., just within that combiner box) contribution of other paralleled strings feeding the fault would certainly be enough to blow a 30A fuse, rendering it safely removed from the circuit. This is because there are 20 other strings in the combiner box.

The client has made the aforementioned assertion, but also contends that since there are also several identical combiner boxes paralleled yet again in the DC combining section of the inverter, there is no risk of damage to the one faulted string, as there will always be sufficient current available to blow the fuse.

Am I just digging my heels in because I've never used these things? His reasoning seems sound to me.

 

[jaggedben]

The client has made the aforementioned assertion, but also contends that since there are also several identical combiner boxes paralleled yet again in the DC combining section of the inverter, there is no risk of damage to the one faulted string, as there will always be sufficient current available to blow the fuse.

This seems like a pretty bogus argument. The logical extension would seem to be that any fuse in the system can be rated at anything up to the total output current of the entire system. Code requirements simply say otherwise.

 

[GoldDigger]

The client has made the aforementioned assertion, but also contends that since there are also several identical combiner boxes paralleled yet again in the DC combining section of the inverter, there is no risk of damage to the one faulted string, as there will always be sufficient current available to blow the fuse.

Am I just digging my heels in because I've never used these things? His reasoning seems sound to me.

Since when have fuses and breakers been required only as protection against bolted faults?
The risk is that there will be a defect in one string that enables it to draw a fault current which is limited by the panel itself and is above the manufacturer's series fuse rating but not high enough to blow the shared protective fuse in the Y configuration.

 

[ggunn]

Since when have fuses and breakers been required only as protection against bolted faults?
The risk is that there will be a defect in one string that enables it to draw a fault current which is limited by the panel itself and is above the manufacturer's series fuse rating but not high enough to blow the shared protective fuse in the Y configuration.

Exactly. For starters, it would draw full string current from the other string in the connector which doesn't even pass through a fuse. Then there's the current through the double sized fuse from the other strings in the array. This is a very bad idea.

 

[GoldDigger]

Exactly. For starters, it would draw full string current from the other string in the connector which doesn't even pass through a fuse. Then there's the current through the double sized fuse from the other strings in the array. This is a very bad idea.

Well, thinking about it some more, I think that there could be two long term outcomes after a current-limited fault:
1. The defective panel/string draws enough current that it catches fire and opens the circuit. No further need for the fuse.
2. The defective panel/string catches fire, shorts out completely and then blows the fuse.
Looks good to me. :dunce:

 

[ggunn]

Well, thinking about it some more, I think that there could be two long term outcomes after a current-limited fault:
1. The defective panel/string draws enough current that it catches fire and opens the circuit. No further need for the fuse.
2. The defective panel/string catches fire, shorts out completely and then blows the fuse.
Looks good to me. :dunce:

Right. It's a self protecting circuit!

 

[Neal30]

I've been following this thread with interest because a client is insisting we utilize these y-connectors for a megawatt+ install.
I have given my objection because of the issues previously pointed out:
1. Module indicates maximum series fuse rating of 15A - (to me, this should end the discussion)
2. Other properly operating paralleled strings in the combiner, (i.e., not one of the strings in the faulted pair), can potentially contribute fault current that is sufficient to damage the module, but not blow the fuse.

To a certain degree I can agree with point 2. However in my scenario this is highly unlikely. If there were a fault, the local current (i.e., just within that combiner box) contribution of other paralleled strings feeding the fault would certainly be enough to blow a 30A fuse, rendering it safely removed from the circuit. This is because there are 20 other strings in the combiner box.

The client has made the aforementioned assertion, but also contends that since there are also several identical combiner boxes paralleled yet again in the DC combining section of the inverter, there is no risk of damage to the one faulted string, as there will always be sufficient current available to blow the fuse.

Am I just digging my heels in because I've never used these things? His reasoning seems sound to me.

I had a conference call with cooper Crouse hinds, I explained my point of view of the series fuse rating of the modules. They all stated (engineers, electricians, designer, sales) that the use of
the tees are more cost effective if the string run is greater than 100 ft. They also stated that you must use in line fuses with these tee connectors. I had a call with an engineer at Suniva, and he had a very good explanation.

When a string of panels is fully shaded it acts as a resistor or heat sink. Normally if this were to
take place the 15 amp fuse would protect this string from damage. When it is tee'd the panel has the potential to draw current from its partner string, and then also draw up to 29 amps from the combiner box it is connected to. I know this would be a rare occurrence, but simply voiding the manufacturers warranty on the modules should be enough to convince someone who is investing a couple million dollars to follow the manufacturers specifications.

The companies that manufacture these string wire sets, will be able to give great reasons for the
in line fuses.

 

[Zee]

I do not know of any in-line fuseholder compatible with PV Wire and the requirements of outdoor, 600 VDC or higher, DC wiring.

"When a string of panels is fully shaded it acts as a resistor or heat sink. Normally if this were to
take place the 15 amp fuse would protect this string from damage. When it is tee'd the panel has the potential to draw current from its partner string, and then also draw up to 29 amps from the combiner box it is connected to. I know this would be a rare occurrence, but simply voiding the manufacturers warranty on the modules should be enough to convince someone who is investing a couple million dollars to follow the manufacturers specifications"

The resistor argument above confuses me. Yes it is a resistor when in shade. It is limited in such conditions to its own single string current.

Are you saying above that the 15 amp fuse should "protect", that is blow, each time shade arrives?
Also, that shade is "rare"?

Why would 29 Amps be the max it could draw?

In a FAULT, any string can draw the max of ALL paralleled strings no matter where they are in the succession of tee's, CBs and recombining CBs.

 

[ggunn]

In a FAULT, any string can draw the max of ALL paralleled strings no matter where they are in the succession of tee's, CBs and recombining CBs.

If there are no string fuses/breakers anywhere, of course. We've been over this ground quite a bit and shown why combining pairs of strings with these connectors and fusing the pairs at 2X the string current is a very bad idea.