Two inverter system

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Two inverter system

Postby loosewheel » Sun Oct 16, 2016 9:16 am

I have just had a solar system installed and am confused about how it's been set up and functions. I have a 9.1kw system (35 x 260W panels) with a 6.5kw inverter (which can charge a battery and feed the grid) and a 5kw inverter (which dumps), and a 6.5kw battery attached. I have an east/west install, which means the real potential of the system is about 7kw. The panels are in 4 circuits, 19 on the east in two circuits connected to one inverter, and 16 on the west in two circuits connected to the other.

In speaking with the consultant from the supplier about the system, I ask for a system that would charge the battery then feed the grid, and draw from the battery before the grid as a priority. The equipment is what he advised for the system.

Looking at the set up now installed I am confused as to whether I have a single 9.1kw system with two inverters set up to charge, feed and then dump, or do I have two independent systems where each inverter is doing with the supply what it does. In the AM the dumping inverter shows greater power than the main one leading me to believe that it is connected to the 19 east facing panels. If independent, does this mean my system has been configured to dump as a priority? Shouldn't the main inverter be connected to 25 panels (6.5kw rating) and the remainder to the dumping inverter? Or do the inverters typically worked together?

Please forgive my rudeness for jumping in with a question. I still have a final payment and would prefer to raise the issue with the supplier before hand, if I have one.

Any thoughts would be greatly appreciated.
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Re: Two inverter system

Postby Gordon-Loomberah » Sun Oct 16, 2016 11:53 am

Welcome to the Energy Matters Forums Loosewheel :)

It's going to be pretty hard for anyone to have a guess at how your system is configured from the information supplied, but your installer should have given you some information about that.

When you say the 5kW inverter "dumps" where does it dump the energy to - hot water, a bank of resistors, or something else? Do you have a limit on how much energy tyou can export to the grid? I'm assuming the battery is 6.5kWh (energy) rather than 6.5kW (power).

What brand and model are the inverters, and the battery? Are the inverters AC coupled?
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Re: Two inverter system

Postby loosewheel » Sun Oct 16, 2016 2:08 pm

Hi Gordon. Thanks for the response.

I was going off memory and got the ratings a little wrong. The equipment:

Inverter Solax X-Hybrid SK-SU5000E (5000W DC, battery connected)
Inverter Solax X1-LX3600 (4000W DC)
Battery LG Chem RESU6.5 (6.5kwh)

The AC coupled I don't know.

I only have a lay knowledge of the technology. In discussion of the system I understood that the system would feed the house, if not using then charge the battery, if charged then feed into the grid. He stated that there is a 5kw limit that can be feed into the grid so if producing beyond that it will just be wasted.

While checking out the system I noticed that when the battery is charging the wattage on the inverter it is connected to fluctuates between a negative few hundred watts and a positive few hundred. But the smaller inverter sits constant, at the time about 1500W. I'm just starting to wonder if we misunderstood each other and this inverter is not feeding the battery, but just wasting because it's connected independently.

I really was looking to check with someone more knowledgeable if it is common to have two inverters working together as a single system to maximise power usage. If so this is probably the case. Or is this unheard of and I need to be speaking to the supplier about favouring the main inverter.

Thanks
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Re: Two inverter system

Postby Gordon-Loomberah » Sun Oct 16, 2016 7:47 pm

AC coupled isn't uncommon, so perhaps yours is connected this way, but it's not really possible to say for sure with little info about it.

Whether there is power going into or out of the battery will change depending on sky conditions, time of day, loads turning on and off, battery state of charge and inverter priorities. If the smaller inverter had a constant output, I'd say it was feeding the grid. Depending on exactly how many panels are connected to it and time of day, sky conditions etc, 1500W may be ok... but without more info, it's hard to say.
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Re: Two inverter system

Postby loosewheel » Sun Oct 16, 2016 11:04 pm

The “AC coupled” is probably what I'm querying. I'll query more detail from the supplier. This has helped. Thank you.
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Re: Two inverter system

Postby APR » Mon Oct 17, 2016 9:27 am

loosewheel wrote:The “AC coupled” is probably what I'm querying.


Hi loosewheel, going by the specifications, the X1-LX3600 is a grid tied inverter. It will be AC coupled to the Hybrid SK-SU5000E. The way I would expect your system to work is as follows...

The LX3600 will produce AC energy which will be used in the house or exported to the grid.

The SK5000E will produce AC energy as will the LX3600, and the SK5000E will also charge the battery pack if the house is not using all the solar production from both inverters. If the LX3600 is meeting house demand, the SK5000E will be able to charge the battery bank at the maximum programmed amperage rate provided the SK5000E is making adequate power from it's own solar generation to do so.

The system should be fitted with either a separate meter or a current clamp on the grid supply, and feedback, from which ever device is fitted, will feed back to the SK5000E to tell it whether energy is going to the grid or coming in from the grid. If energy is coming in from the grid the SK5000E will use battery power up to it's maximum capability to limit imports from the grid. If energy is going to the grid and you are export limited, the SK5000E should be configured to limit it's output to control power going to the grid.

If you have been export limited to say 5kW, your battery pack is charged, your house is using very little energy, and your solar system is producing it's maximum, what will happen in that the LX3600 will produce all that it can, and the SK5000E will limit it's power output so no more than 5kW is sent to the grid. Note, you may not be able to limit power from the LX3600, but it should be sized so it alone cannot exceed the export limit.

What I do not know about your SK5000E is whether it can use it's solar generation as well as excess AC generation from the LX3600 to concurrently charge the battery pack if solar generation from the SK5000E is inadequate to charge the battery pack at the programmed amperage rate.

I am in the process of doing similar to what you are doing, and my approval has placed an export limit of 5kW on my system. I am using 2 hybrid inverters (different brand to your inverters, but same basic operation) so I have a backup hybrid inverter in case of failure, however, my battery pack only feeds one inverter, and my second inverter will be doing exactly what I expect your LX36000 will be doing. Below is an image of my connection layout which may give you a better understanding to confirm with your installer your system configuration. Your system might be using a current clamp to feed info on grid power level and flow direction back to the SK5000E instead of a meter...

Image
2 x Goodwe GW5048D-ES Hybrid inverters, 13kW panels, 20kWh LiFePO4
Mk2 PV Router feeding excess solar generation to a 3.6kW 250 ltr HWS
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Re: Two inverter system

Postby loosewheel » Tue Oct 18, 2016 5:21 am

Hi APR. I have been in touch with the supplier and he said the X1-LX3600 is not tied to the grid. That when the X1-LX3600 is below its potential the SK-SU5000E is fully available to charge the battery and feed the grid, which he states is connected to the 19 east panels. And considering my usage, excess from the X1-LX3600 will not be significant enough to justify the extra expense necessary to push it to the grid. He needs to come back to fit some labels on the equipment and I'll speak further with him then.

Thanks
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Re: Two inverter system

Postby davidg » Tue Oct 18, 2016 8:26 am

APR wrote:
loosewheel wrote:The “AC coupled” is probably what I'm querying.

Image

The problem I see looking at that diagram is that this installation is "hybrid" install as defined by the CEC nowadays. What that means is if the grid fails then the system will drop offline as well.

If that's definitely not the case and the system supports grid failure for sure then the diagram is incorrect and or incomplete.

If the system was sold as "load shifting" hybrid system and you are aware that should the grid fail that you will also be in blackout as well then fine otherwise ........................

This could be the Here we go, start of systems, where endusers don't or didn't realise that it will not work during grid blackouts. I've had extensive conversations with other sparkies about this, we're waiting to see the fallout from less scrupulous sales people selling battery systems or retrofits and deliberately neglecting to let end users know this particular aspect of various types systems on offer.

The STD Tesla Powerwall battery definitely falls into this bracket, no grid no power at all until the grid is available again.

I suspect there's going a be to lot of end users that will only find out after, what they actually paid for is not what they thought they paid for.

Have the system been tested to see what happens if the main switch to the grid is turned off?
N.B turn off any devices that you don't want to loose power to suddenly should be shutdown first, JIC.
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with AC-Coupled Grid-Connect Inverters,
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Re: Two inverter system

Postby APR » Tue Oct 18, 2016 10:28 am

Hi Davidg, The system needs either a manual or auto transfer switch for when the grid goes down, and you need to activate Off-Grid Mode by switching the configuration via a mobile device app.

When in Off-Grid Mode or Back-up Mode (UPS mode), power is taken from a dedicated port (UPS/Off-Grid port) on the inverter, not the port that connects to the grid. The transfer switch has to disconnect the house from the grid, and connect the house to the UPS/Off-Grid port. The inverter has several operating modes as seen in the image below.

Image

A limitation of using two of these inverters is that the UPS/Off-Grid outputs cannot be connected to supply a common circuit, and, being transformerless, they do not have the surge capacity of the heavyweight off-grid inverters such as the Sunny Island or Selectronics off-grid capable range.

In General Mode the battery pack is charged off solar, and power is drawn from the battery back when solar is insufficient. The grid takes over once the battery pack reaches a programmed SOC lower limit.

In Economical Mode, the inverter can be programmed to charge the battery pack from the grid between programmable hours each day, and to also send power from the battery pack to the grid between programmable hours each day. The inverter doesn't have programmable pins which can be use to actuate these activities like, for example, the Selectronics range has.

All configuration changes are made via an Android or Apple iOS mobile device, and the inverter is the Goodwe GW5048D-ES. (The Android App is OK, the Apple iOS App needs more work)
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Re: Two inverter system

Postby davidg » Tue Oct 18, 2016 10:39 am

I see,

that would be just total a pain, for lots of people.

I much rather use an Selectronic SP-Pro (Which I have here, see my signature) thanks it does all that and more, it's simply programmed and assuming the batteries have sufficient charge which they should, will always by default go to batteries for grid fails in most circumstances you don't even know there's been a power failure or interruption to power at all.

Using a phone app ................... hmm if there is a blackout how do you communicate with the system unless it's direct which also requires you to be at site/home at the time and know how to do it. I think that's a bit of clunky solution personally. #just saying.
Off-grid Systems specialist- You could store Sun to use later- I Do!
SELECTRONIC SPMC-482 7.5kW, 8.2kW's of Arrays

with AC-Coupled Grid-Connect Inverters,
1100Ah 48V Bank

Auto Backup Genset
PVOutput Stats

Sparkys light up your life :)
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