Reality Check on Solar Reconfiguration

System · January 2010

Good Day,

I am building a solar array and trying to maximize the current and voltage from the array to a Trance ST1500 grid-tie inverter. The inverter uses MPPT and achieves full output between 52 – 85 Vdc. Anything less than 42 Vdc the inverter won’t kick on and anything over 85 Vdc is just wasted.

I currently have 6 – 180 watt panels with a Vmp of 25.9 and Imp of 6.95. What I’m wondering is if I add a 120 watt panel of 17 Vmp and 7 Imp in series with 2 of the 180 watt panels, will this work?

If I have done my math right, I should wind up with a combined output of 68.8 Vmp (25.9+25.9+17) at 6.95 Imp?

I am asking because I have not found a post that seems to address this specific concept. Before I head down this path, I want to make sure I have it right and not a recipe for disaster.

Any thoughts, suggestions, or comments are greatly appreciated.

BB. · January 2010

Re: Reality Check on Solar Reconfiguration

I guess this is a "Trace" inverter (aka Xantrex, aka Schneider Electric)?

Is this an existing setup or an new installation?

In general, you can put in series solar panels that match Imp within ~10%.

And put in parallel panels that match Vmp within 10% too (more than 10% miss-match and you start losing significant amounts of power and may have other issues too).

Were do you live/have the installation... Generally, you aim for Vmp>Vin-minimum on the hotest day you expect the run the system (hot panel means low Vmp).

Voc is 120 VDC on the system -- so you want to make sure the Voc is less than Vin-max (on your coldest day) to avoid damage to the inverter.

It is an old system so adding arrays and/or doing a "new install" may not be worth the issues (no repairs? may not meet current code?).

The current crop of pure GT inverters generally run at high voltage (200-600 VDC) which can be an issue for older panels (not UL listed to 600 VAC).

-Bill

Solar Guppy · January 2010

Re: Reality Check on Solar Reconfiguration

Is the Suntie a true ST or the or a XR-UPG version , you can tell by the firmware version, 5.0 is the upgraded inverter, the original ST1500's are worthless and have all sorts of tracking issues. Try and keep the vmp as low as you can, the higher voltages are a real issue with the original ST's, but you need 60Vdc be have the inverter wake up

System2 · February 2010

Re: Reality Check on Solar Reconfiguration

Good Evening BB and SG,

My apologies, I keep throwing the "n" in there for some reason. It is a Trace (Xantrex) ST1500. I am not sure of the firmware off the top of my head; I will have to wait to check.

In general, this has been a work in progress over the past year. The setup has been inspected by the local electrical inspector and signed off on by the local utility company. The panels are about 1.5 years old and were purchased new.

Therefore, if I read the comments below correctly, the main point is to keep in mind is that the Imp needs to be with in 10% of each other for the best power generation. So, if I were to connect in series the three panels ( 180W, 25.9 Vmp, 6.95 Imp + 180W, 25.9 Vmp, 6.95 Imp + 120W, 17 Vmp, 7 Imp ) this does not present a problem (i.e. damaging the 120W) for the panels? Or for that matter the inverter? As indicated, I just want to make sure I’m clear on this point. I was just looking to fine-tune the system to make sure I am in the sweet spot of the inverter.

Thank you for your time.

FreeWatts · February 2010

Re: Reality Check on Solar Reconfiguration

As BB said earlier
"And put in parallel panels that match Vmp within 10% too (more than 10% miss-match and you start losing significant amounts of power and may have other issues too)."

BB. · February 2010

Re: Reality Check on Solar Reconfiguration

Not a problem with the series string current at all... Ideally, you want to confirm that the solar panels are rated for >120 VDC connection (some panels are as low as 70 volts. Most of the other larger panels (like over 100 watts) if UL/NRTL marked should be approved for 600 VAC max connections.

Otherwise, the Vmp voltage looks OK too... (not too high, not too low).

You probably will have to put 120/240 VAC to the inverter to read the firmware revision.

-Bill

System2 · February 2010

Re: Reality Check on Solar Reconfiguration

Good Evening BB and FW,

Thank you for the follow up. As indicated I figured double-checking was just better off than finding out the hard way.

BB – I did not answer you question about location and weather. I live in PA and the average temperature is 20 deg F at the lowest point and 83 deg F at the highest. Of course, there are outliers (we can hit negative numbers at time and have hit a 99 or a 100 deg F at one time or another).

I think SG brought up a good point and you touched on it as well. With the new inverters that are out with the higher voltage requirements are there any thoughts on a good upgrade for the ST1500 (didn’t make it home in time to check the firmware) when the time comes?

Thanks again for the assistance.

BB. · February 2010

Re: Reality Check on Solar Reconfiguration

Our host is a solar retailer/wholesaler stands behind their products--so you can start here:

http://store.solar-electric.com/gridtiesolar.html

The one vendor with a, possibly unresolved, issue (that I am aware of) was a thread where a person was having an issue with a Fronius GT inverter (some string sizing/configuration and output voltage range issues that they where unable to resolve for the poster).

Fronius MPP Hunting

I am not in the "solar business" (not our host nor any vendor/installer)--So, regarding the above issue--all I know about is what is in the linked thread.

-Bill

Reality Check on Solar Reconfiguration

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