Mixing Xantrex XW and GT ?

aj164 · January 2009

What is the main difference between having a complete Xantrex XW system (with the PV coming in through XW MPPT controllers) versus PV going through separate 240VAC GT inverters?

In other words, what happens if I use the XW system as a big UPS (batteries only, no PV input), and I connect my PV array to the AC side via a GT inverter? It seems like I would get better PV conversion efficiency under most conditions, except when recharging the battery bank after a discharge.

I'm thinking of the conversion efficiency, assuming a charged battery bank, going from PV DC (130V) via MPPT (x%) > Battery V (48V) via Inverter (93%) > 240VAC .
Whereas for the GT system, the conversion is simply PV DC (350V) via Inverter (95%) > 240VAC.

I believe the GT will track the XW output, so I should still be able to harvest PV when the grid is down. BUT: If the AC loads are less than the PV output, will the batteries charge through the XW inverter?

If I'm reading correctly, the XW inverter requires a minimum load before it will start. So let's say the grid goes down. The GT inverter output stops. The XW transfer switch engages, and the XW inverter senses a load and starts up. In 5 minutes, the GT inverter comes back online. (still no grid) At some point, say the GT output is greater than the loads in the home. Assuming the XW will then top up the battery bank, what happens when the batteries are charged and there's nowhere for the excess PV output to go?

BB. · January 2009

Re: Mixing Xantrex XW and GT ?

Hopefully, Solar Guppy can come by and give the correct answer...

AJ, if I understand your questions correctly:

1. Is not the XW system (panels to charge controller, to battery, to GT capable inverter to AC mains) less efficient than just going GT (panels to GT inverter to AC mains, charge batteries with AC power).

I would guess you are correct... As I understand, the XW GT inverter looks at the battery voltage and "turns on" when the batteries are fully charged.

I have not seen a detailed explanation of how the XW GT inverter works, but it would be great if it's output power (rate) to the Grid was somehow porportional to either the battery voltage or the charge current as measured by the XW solar charge controller--this way, you will not be losing power "micro" charging/discharging the battery bank (still have Solar Charger + XW GT losses).

2. What about connecting a pure GT Inverter to the XW Hybrid GT inverter... And what happens if the AS Mains Fail. What happens if the batteries are full when the Mains are down.

The short answer is that yes, you can connect a GT Inverter and an XW hybrid GT inverter together when the mains are down. The GT inverter will pump energy backwards into the XW inverter and charge the battery bank. This appears (from what I have read) to work fine--although, the manufacture probably does not support this type of operation (some other True Sine Wave Inverters can also charge the battery banks by being "back driven" by a GT inverter too). And this is a full "automatic" function... The GT and XW inverter will properly share source power and loads without any problems on the AC loads.

The problem arises when the battery bank is fully charged. There is no regulator in the GT or XW system to regulate the power going into the batteries. Worse case, the batteries will boil dry, overheat, and possibly catch fire and explode the hydrogen in the batteries and the basement.

So--there you have the problem. We have talked here about how one could regulate the battery bank to prevent overcharging... But, 1. you need to know what you are doing; and 2., you probably would need support from Xantex to make sure that your added components work with the existing the XW hardware/software and does not fight the XW's several battery chargers (probably not going to happen with an individual customer).

In the end, at this point, if you need of a grid capable GT/utility interactive system--I would not recommend that you go through all of the extra work to save ~5% of "extra losses". In the grand scheme of things--you would have a hard time even measuring the losses vs the issues/costs/dangers of trying to build a hybrid / hybrid system.

Did I answer your questions clearly (at least to the best of my information/ability).

-Bill

PS: To be clear, there are two places where the "Pure" GT solar powered inverter can be connected to the AC mains:

A: In "front" of the XW system--I.e., directly to the AC mains. The GT Inverter will work just normally. If AC mains are on, the inverter will convert. If the AC mains fail, the GT inverter will turn off and wait until the AC is restored (5 minute timeout). The solar panels are useless when the AC mains are down.

B: Connect the pure GT Inverter on the XW Hybrid GT Inverter's battery backed output. When the AC mains are up, one (or both) GT inverters will push power out to the AC mains. When the AC mains are down, the XW H-GT will behave like a UPS and simply disconnect from the AC mains and power the local loads. The P-GT inverter will also see "AC power" and continue to convert (outputting as much power as the solar panels allow). The XW H-GT will either make up for any additional needed power, sharing the load with the P-GT; or the XW H-GT will such up the extra power and use it to charge the local battery bank--and this is where the issues begin.

-BB

Ecnerwal · January 2009

Re: Mixing Xantrex XW and GT ?

Simple engineering answer - the cost of buying both GT and XW inverters to do what XW will do all by itself will easily cover the cost of an extra panel or two to overcome any perceived or actual inefficiency in the XW. Plus, it will pass inspection, which I'd guess the complicated mess won't.

Not unlike the rule of thumb that a good, low-cost energy-star fridge plus a couple of panels beats an expensive sunfrost in total dollars input and function...

System2 · February 2009

Re: Mixing Xantrex XW and GT ?

The SMA Sunny Island inverters are designed to do what aj164 wants to do but only within their own family of GT inverters (eg: Sunny boy/windy boy) and at what seems to be a premium price.

I already have a Sunny Boy and solar panels. To add a battery-based system like the Xantrex XW (or an Outback Power system) I have the same problem/question. I think BB is correct about the choices. I agree that it is probably dangerous to choose option B. The problem is clear - if your GT inverter is pumping out more power than the current load, where does it go if the grid is down and the batteries are topped up? The SMA solution is a communication from the Sunny Island battery inverter to the other load-side sources telling them to reduce output. Easy enough for a solar inverter. A wind or hydro power controller probably has to dump to its diversion load/heater.

So why not just choose option A? Well, if the grid is down, your GT inverter on the line side will shut down. I can live with that, but only because my grid power is reliable enough. If your's isn't then maybe you have a different viewpoint. Any other problem? Well, if I have PV on my GT on the line side and wind or another PV array (or hydro) on the battery system they both may want to sell at the same time. Each one has to raise the line voltage slightly to do so. Can this cause the line voltage to go out of spec. for one inverter or the other? I do believe that multiple Sunny Boy's (for example) can coexist like this being from the same vendor, but if you have inverters from two different vendors? Not so clear perhaps.

Ecnerwal is certainly right too - unless you are considering expanding an existing system and adding battery backup. Pragmatically, battery backup is not cost effective. Cheaper to have a limited battery capacity and a generator. Not so "green" perhaps. The real reason I am considering batteries now is to add wind power. Most small wind turbine alternators put out voltage for battery charging. Utilities and local electrical inspectors may also be nervous, particularly if you try to hook up a home-built turbine via a windy-boy (for example). My suspicion is that a wind turbine hooked to a battery bank and then to the grid via a listed inverter might be easier to get past inspection.

BB. · February 2009

Re: Mixing Xantrex XW and GT ?

ewb,

Welcome to the forum...

If I understand your question--You sound not have any problem with a Xantrex GT selling AC, and a second (Sunnyboy, Windyboy, XW hybrid) selling AC to the utility either--assuming your local wiring, breaker box, and feed back to the utility pole meet code/design requirements.

If your X GT + SB/WB GT + X XW all add up to more than the NEC limits (or your AC branch circuits are too long/too small of gauge and end up with high voltage drop)--then you will have code/operational issues.

You can use the wiring calculator (or the online calculators/wiring resistance charts) to double check your voltage drops and make sure they stay under 3% total voltage drop--at max current (I think that is the NEC requirement--but I am not sure).

-Bill

System2 · February 2009

Re: Mixing Xantrex XW and GT ?

Yes you are right. If I were trying to sell enough current to cause a significant V-drop to the transformer then one or more of the selling inverters may see an overvoltage. This could be caused by inadequate wiring between my inverters and the transformers (too much local impedance). It might get more interesting in the case where the inverters are not all close together and thus see different impedances. I'll avoid that.

I think a likely result is one or more selling inverters stops selling which relieves the situation. Then 5 minutes later they try to sell again - over and over. Or maybe they are smart enough to give up after awhile?

I guess it didn't occur to me that NEC and/or UL1741 or other standards may address the question. Time to dig into that a bit. Thanks for the pointer. I'll also look into how much my Sunny Boy raises the line voltage when it starts to sell.

BB. · February 2009

Re: Mixing Xantrex XW and GT ?

No, the UL standard is just a safety standard... It does not (as little as I know) address "current sharing". They will retry every five minutes until a) the sun goes down or b) when the power is reduced enough to keep everything in spec.

If you are having problems with your line voltage going out of range... Check the voltage without the inverter running, and again with the inverter running.

In theory, you should only see a 3% increase in voltage (240 to 247.2 VAC) between 0 and 100% rated branch current.

If your zero current is way high (~260+ VAC)--then you need to check with your utility.

If your voltage starts low, then peaks (start at 240 VAC, then the inverter lights off and you get 264+ VAC), then you have either insufficient house wiring or a poor/corroded connection (breaker panel, inverter panel, wire nuts, etc.).

-Bill

System2 · February 2009

Re: Mixing Xantrex XW and GT ?

On 2/9 I had a very short AC glitch (< 10 seconds) that shut down my Sunny Boy for about 6 minutes. All normal. It was bright and sunny and cool (below freezing) so I was generating near max amps (about 8.5A at 240V, or 2000+ watts). My PV array is rated at 2250 peak watts. I monitor the Sunny Boy with a laptop that has batteries so it doesn't crash when the power crashes. I pulled up the data across that glitch and I saw my mains AC voltage at 243 volts (RMS) during the 6 minutes the inverter was not pushing power and it leveled off at 245 volts after the inverter resumed selling so it looks like the inverter was pushing the voltage up by about 2 volts.

I can only estimate my household load at the time - probably somewhere between 500 and 1000 watts. So I was probably selling between 1000 and 1500 watts, or 4.2 amps to 6.2 amps. Which means my impedance to the utility "source" is between 0.47 ohms and 0.32 ohms.

Call it 1/2 an ohm and say that my nominal mains voltage is 250 volts. That leaves me with 14 volts of headroom before GT inverters shut down at 264 volts. 14 volts across 1/2 ohm is 28 amps or 6720 watts at 240 volts (All rounded in the conservative direction). So I ought to be able to sell that much power.

What do you think of this analysis? The point here of course is to decide how much headroom I have for more PV and/or wind power through additional GT inverters.

BB. · February 2009

Re: Mixing Xantrex XW and GT ?

That is how it is done... Change of Voltage/Change of current = Resistance...

Of course, some of your power is going through a dedicated branch circuit (from the inverter to the breaker panel) and shared current (breaker panel through rest of home and out to transformer.

The other requirements (125% maximum Panel capacity--Main Line in amperage + grid tied inverter amperage =< 125% of panel capacity).

Interestingly, reading through the information posted by Jim/Crewzer earlier, if the AC Main line was coming in at the top of the bus bar, then the GT power breakers had to be installed at the bottom of the bus bar--so that no place on the bus would see more than 100% of rated current.

Interesting requirement--I had not seen it spelled out before that the GT and Mains bus bar connections could not be at the same end of the bus bar.

Of course, then I could wave my hand and say that the above should be 200% of box capacity--because the sources are at either end of the bus and no point will see more than 100% of capacity (actually, the UK has exactly that type "ring" wiring system. To sources that meet at the outlet--and why UK plugs have to have a fuse in them to prevent 2x rated current through the cord.

NEC still does not always make sense.

-Bill

niel · February 2009

Re: Mixing Xantrex XW and GT ?

that 2v rise is interesting and i'd be curious what others get without the gt suddenly connected? i would think it shouldn't rise quite that far normally, but i could be wrong.

System2 · February 2009

Re: Mixing Xantrex XW and GT ?

In my case the dedicated branch circuit from the inverter to the panel is very short - 4 feet or so on 10 AWG, and virtually all of my load current goes one way - to a sub-panel that feeds the house while current sold back to the grid goes another way. Yeah, if all the current traveled a substantial distance on a shared sub-feed then you'd be doing more guessing. I suppose the right way to do this would be to shut down all the loads and then momentarily shut down the inverter and capture the line voltage both before and after it started selling again.

My back-fed breaker from the inverter is at the bottom of the bus bar - totally by chance

I agree that 2 volts seems like a lot. By my back of the envelope calculation I am thus limited to selling maybe 7000 watts. I too would be interested in knowing what other people see. I have probably 250 to 300 feet of drop from the utility transformer to my inverter which is probably typical. The first half or so of it is on utility aluminum - presumably 4/0 for 200 Amp service. The second half is on 3/0 copper. All buried. I also have more than the usual number of breakers in the sell path - uhmm 4 I think.

Also these numbers are based on what the inverter told me in a data capture every 10 seconds. Its resolution is one volt (it doesn't report fractions of a volt). Obviously it would be better to do a controlled experiment and use a meter with another digit of resolution and known accuracy. Clearly you need good data to discriminate 1/2 ohm from 1/4 ohm.

BB. · February 2009

Re: Mixing Xantrex XW and GT ?

If the loads are stable--you don't need to turn them off. The change in voltage and change in current is all you need:

V=I*R
R=V/I=delta V / delta I = 2 volts / 8.5 amps = 0.235 ohms

264-250=14 volts of nominal head room

I=V/R=14V/0.235Ohms=59 amps

59 amps * 264 volts = 15,500 watts or 15.5 kW

15,500 watts / 240 VAC * 1/0.80 branch circuit = 80 amp branch for solar

Assuming that your solar current+panel capacity cannot be more than 125% of panel rated power:

80 amps / 0.25 = 320 amp minimum utility service / breaker panel capacity for a ~15,500 watt PV GT System

Anyway, those are the rough requirements/capabilities of your current service (the above is just a guess on my part--I am not a licensed electrician/solar installer--based on your interesting measurements). You would probably need to step up to the next standard service size (320 is not standard--I am sure).

15 kW is not a small (or cheap) PV system. Do you need that much power? Will your utility pay you for excess power?

-Bill

System2 · February 2009

Re: Mixing Xantrex XW and GT ?

With refrigerators and furnaces and (not in my case) A/C units operating on thermostats there is no guarantee of load stability across a 5 or 6 minute interval. The only easy way to get that is to just shut the whole house down, which is no big deal. I do agree though that *if* the load was stable across the measured interval then I should have used the whole 8.5 amp delta, resulting in a lower resistance than I calculated.

No I can't go to 15.5kW of renewables. Under the "standard offer" from Xcel in Colorado, residential systems are limited to 10kW nameplate. Yeah you get the retail rate from Xcel (currently) single net-meter. I'm only considering an expansion of my 2kW of PV with an additional 3 kW of PV and maybe later a wind turbine on the order of 5 kW max nameplate.

Mixing Xantrex XW and GT ?

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