Trouble managing dual XWpro AC2 input limits and battery charging

SolaRevolution

I'm working with a system with:

(2) XW Pro 6848 NA inverters, (4) discover 7.4 LFP batteries, and a 15kw genset. (Solar too)

The owners have an air conditioner (3-5ton) which works fine with only the inverters OR with the generator only (in bypass). 

The trouble comes up when the generator comes on while the A.C. is running. The client called reporting a problem that the generator was running but had no output. I got there and found the generator main breaker had tripped.

I did my initial set up with the help of tech support and set the AC2 input limits to 50 amps (50*240 = 12,000) I was under the impression that these limits were per system rather than per inverter and this would provide ample headroom for surges. It seemed to work well through the cooler months with lots of household loads, where the system was working properly and protecting the generator by reducing the charge current when needed to prioritize loads.

I've got both inverters set to 100% charge rate, so the max charging rate should be no greater than 13,600 watts - a great high current to reduce generator run-time, but it becomes critical to limit the maximum current that can be drawn from the generator via the AC2 inputs. I have observed the chargers running at around 12k - 13k, and I've seen it throttle back the chargers but the response time can be slow, sometimes allowing the draw from the generator to spike significantly - thus the tripped gen main breaker.

Schneider tech support has told me that setting the AC2 input Amps limits is per inverter, so if I want to limit the draw from the gen to stay below 14kw (14,000w / 240v = 58.33 amps / (2 inverters) = 29.16 amps) I need to set the AC2 input limits to 29 amps.

However, when I decrease the AC2 limit to 40 amps (supposedly per inverter) the charge rate drops to around 9k. If I set both inverters to 29 amps the charge rate decreases even more, thus increasing generator run time.

Schneider tech support has said that some settings are per inverter while others are per system. In spite of what tech support says, AC2 input limits seems to be per system - at least in how it limits the chargers.


 I'd like to keep the charge rate as high as possible while protecting the generator from damage caused by over current. I'm wondering if I created a conflict by allowing the 100% charge rates to exceed the AC2 input limits.

 Does anyone with experience with these settings have any useful insight or suggestions?

RCinFLA

There is a finite reaction time for inverter to supplement AC input.  Sometimes a large output AC surge will make it through to the AC input for a few milliseconds before inverter reacts to supplement AC input.  This can cause a momentary overloading on generator.  If generator bogs down its output frequency will shift.  The inverter cannot track a fast-changing input frequency and will release from generator.

When you also have a heavy charging load from AC input, the inverter control feedback also has to immediately cutoff charging current to allow inverter to supplement AC input current limit.  Battery voltage is going to quickly shift from a charging overpotential voltage to a heavy discharge slump requiring inverter PWM feedback adjustment to maintain AC output voltage.  All this takes a little reaction time allowing some momentary bleed through to generator loading above the set AC input current limit.

You should try to avoid heavy charging when you need the AC output startup surge current.  You may also have to play with limiting AC input to a lower value which may help in the reaction time to supplement AC input without putting too much bleed through surge current on generator.  If you hear the generator rpm speed bog down you will likely get inverter releasing from generator.

Battery voltage x charge current = charging power, divide by inverter efficiency = AC power, divide by AC voltage = AC input current for charging. i.e, 51v battery x 78.4 amps charging current = 4,000 watts / 0.91 effic. = 4400 watts AC power / 240vac = 18.33 AC in amps for charging.

AC input current limit effects the total current taken from AC input for both combined sum of battery charging and AC out loads.  It will automatically back down charging if AC output demands more of the available input power.  If AC out current exceeds AC in limit then inverter/battery makes up the shortfall.

If air conditioners are traditional single phase induction motors the starting surge can be greatly reduced by booster-softstarters like MicroAir EasyStart 368.  I use one on my 4 ton central air conditioner to run it on my inverters.  You will typically get 60-70% reduction in startup surge current.

Dave Angelini

It is a huge mistake to try and run a conventional AC for an offgrid app. There are so many better choices with Split type systems that soft start.  Adding in the 3 ton Offgrid, and a genset is a headache that you should not do to a customer, in my opinion.