Good morning all, Have a quick question for you guys here, as what I experienced yesterday goes against what I thought I know about my system.

I would attempt to provide as much info as possible here to try and cover all other role players in the system.

Due to the inability of the people elected into office to provide continuous stable power, I had another outage yesterday during the day.

Inverter is running in battery mode, so Eskom is not supposed to be in play.

Loads are run from PV/Battery except in a case where there were no/little PV and the batteries drain below the desired SOC, then the inverter would switch back to grid.

The first image below shows an overview of what happened. The purple line shows that grid fell away for the period in question.

The part that is bothering me however is the PV that dropped by approximately 100W during this time.

The load stayed constant So I cannot see a reason for the PV to reduce. If it was a Grid Tied system and I fed into grid, maybe, but in this case it is not.

Second picture is just a zoom to the area in question.

Battery Amps from plus 1Amp to minus 2 Amp then back to 0A and up to 1 Amp after about 10 minutes. (assume float).

Battery Volts drop by 0,45 Volt and when Power comes on/PV recover climb by 0,2 Volt before resuming.

Load Watts run with a positive and negative swing of 3 Watt either side, so 6 Watt in Total.

PV Watts drop by 50 Watts.

PV Volts drop by 15 Volt.

PV Amps drop by 3Amp.

Grid Volts drop to 0 Volt

Grid Watts stay at 0 Watt. (Obvious)

So my question in short is what influence could the Grid possibly have on the system if no grid is used anyway?

I intend on manually dropping the Grid on Saturday to replicate this, as I do not believe this was coincidence. The values vs time follow to perfect for coincidence.

Thanks in advance.

R

Depends on what inverter. If it is a grid-tied/hybrid inverter, and say you have a grid limiter installed and you're feeding back to some of your loads grid side, then when the grid fails those loads go away. It also sounds as if the batteries are full. So when the grid fails, the PV has to be throttled to match your loads. So this might be completely normal.

Thanks Plonks

Standard Axpert 3kVa Plus, so nothing fancy.

No other fancy stuff, Power low end loads aka lights, entertainment and connectivity.

Only reason it has connection to grid is if batteries go down as result of continuous bad weather to power from grid.

Last current and watts from grid was months ago.

Even if the batteries are on float aka "full" with the PV providing lets say 1 Amp to batteries to keep them float and 5 amps for loads, in my view this supply should continue irrespective of Eskom not?

Thanks

R

Edited September 13, 20186 yr by Riaanh

25 minutes ago, Riaanh said:

Standard Axpert 3kVa Plus, so nothing fancy.

OK, then I do not know :-)

It's possible that the grid was supplying the losses via a small AC power supply. And that the thing that they are calling PV watts is actually PV watts into the battery.

Perhaps check the PV current going into the inverter during the day when supplying loads. Then open the AC input isolator or breaker, to simulate Escom going down. See if the PV current actually goes up, down, or stays about the same.

My guess is that it actually goes up, especially since the PV voltage goes down. So this is just a mislabelling of the PV current. From memory, the 5 kVA models do something similar. 

Thank you for taking the time to look and comment @Coulomb

I will try on Saturday when at home during the day and report back on Monday.

2 hours ago, Coulomb said:

It's possible that the grid was supplying the losses via a small AC power supply. And that the thing that they are calling PV watts is actually PV watts into the battery.

Just want to make sure I understand this correctly @Coulomb. 

In laymen's terms. Grid was powering the unit ( supplying the losses) , when PV took over, PV had to power the unit, but the the PV watts only gets measured after self consumption? (Or something like that)   

 

On 2018/09/13 at 8:10 AM, Riaanh said:

The part that is bothering me however is the PV that dropped by approximately 100W during this time.

Just another guess, What is you efficiency for DC-DC conversions and then DC to AC conversion.  While Ac was percent, the MPPT had to take a higher DC voltage and convert that to a lower DC voltage for the Battery's. Easier work , lower losses.

When Ac failed, the mppt had to take the same voltage and convert it to a higher AC voltage, More work, higher losses, less watts?

Just a question.. no tot sure.. 

Can not be, Its not a Hybrid.. Either AC of PV, not both.. 

 

15 minutes ago, Jaco de Jongh said:

What is you efficiency for DC-DC conversions and then DC to AC conversion

Thanks Jaco

I will need to investigate and confirm. I plan to monitor everything inline as well to ensure all values correlate. I know the mppt gets tricky as the PV Current and the current going to the battery differ significantly. I will test all and see if I can find the source. Admittedly not a train smash, but would like to know as it resist all common reason.

1 minute ago, Riaanh said:

I will need to investigate and confirm

DC - DC        = 98% 

DC to AC     = 90-93%

 

1 hour ago, Jaco de Jongh said:

In laymen's terms. Grid was powering the unit ( supplying the losses) , when PV took over, PV had to power the unit...

I think my theory doesn't hold up to scrutiny, sorry. The Axperts have to calculate battery current, as there isn't a sensor to measure that. They assume fixed efficiency, and measure PV power and AC output power. It's possible that for a small change like this, the assumptions aren't valid, and there is a small error.

Ah, I've just realised that the models smaller than 5 kVA have a separate AC to DC charger; they don't use the inverter in reverse as the 5 kVA models do. So they won't have the small power supply that is capable of running just the display and electronics; it will be an actual battery charger capable of at least 10 ADC. That may have been "idling" when grid power was available, and when the grid goes down, that "idle" power disappears. I just don't know how the lower power models work for AC charging.

I think it will still be interesting to do the measurements.

6 minutes ago, Coulomb said:

The Axperts have to calculate battery current, as there isn't a sensor to measure that.

This rings a bell. When we did some development for Axperts we had to do some of the calculations as the Axpert did not have all the data.

Our external calcs in SQL where more realistic, as we brought in the BMV's data, and some clever maths.

Am under correction, as it has been a while ago ... 

1 hour ago, The Terrible Triplett said:

This rings a bell. When we did some development for Axperts we had to do some of the calculations as the Axpert did not have all the data.

Not exactly the same thing .The Multi is similar: There is no current sensor on the battery side, no Hall sensor. It only has sensors on the input and output, and it can calculate how much goes out the third leg (to the batteries) by doing a simple subtraction sum between these two, and then multiplying it by an efficiency factor. It assumes this factor is fixed (usually 93%) but that is of course not the case. So any measurements under 2A is not accurate, and under 1A basically just noise.

Thanks for all the feedback, I see a busy weekend of metering on the horizon

Will report back.

During the day the pannels supply power to the inverter and it charges the batteries and supplies AC to the house. During charge or a full battery state there is no load on the batteries so their voltage is slightly higher as they run without a load. If Eskom goes down then there is a slight draw on the batteries and thus the loss in power reported. The battery voltage is taken as a refence voltage through the charger and so with a load( P=IV )that would explain the drop in power. Shortly after when they go back into float the power levels should be almost the same

Just some feedback from the weekend. Still have not isolated the issue, but will hopefully soon.

1. There are NO current flow or watts going to the inverter when in the running from battery state as is the normal setting on my side.

2. When the batteries are "Bulking" charging, and I drop Utility side, there are ZERO impact on the PV produced, PV Voltage, Current and obviously Watts stay constant, or within 5 Watts either way.

3. In the afternoon, when the batteries "float" and I drop the Utility it happens again, PV drops as per Initial post.

4. Still no current draw from utility just before I dropped the Utility.

I will continue to measure more things, But a weekend is only so long.

Thanks

Riaan

On 2018/07/20 at 4:45 PM, The Terrible Triplett said:

Just in, Engineers cost for signing off a grid-tied system, this is fair.

Qualifications: Pr.Eng. , MSc. Eng (Electronics)
Fees:
Band 1: Fixed fee for systems smaller than 2.5kW: R 2 500 (R 2 875.00 incl)
Band 2: Above 2.5kW the cost is an additional R0.50c per Watt up to 10kW. (R 4 312.50 + R 2 875.00) 
Band 3: Per additional Watt above 10kW it lowers to R0.20c. (etc)
So a 10kw system would cost R 6 250 + VAT = R 7 187.50 incl VAT

Now that makes sense to me, not a flat R10k stock standard price irrespective of the size of the system, or whether it is on/off grid.

One still has to sort the CoC before engineer arrives.

If you want the details, PM me, for it is not a good idea to leave salient details wide open.

 

Note: Off-grid needs no engineers report, I quote CoCT email:
For an off grid system you need to complete the off grid declaration form attached and provide CoC. The City will investigate to determine if the system is off grid if so, provide an approval letter. There’s no need to appoint an Engineer for an off-grid system.