ldalziel avatar image
ldalziel asked

Feeding excess solar back to grid

Firstly as an overview I have a system that was initially based on a professionally installed conventional solar grid connect inverter which was used supplement daytime power usage for my home office (Without the solar typical week day usage was about 30 KW hrs per day) Although based on only 2.5 KW of panels, I found that I was still feeding to the grid 2-5 KW Hrs per day - during 'daylight' periods. Overall though I halved my grid demand. I then installed a 5 KW Victron Multiplus, Color Controller and 10 KW hr BYD battery system. (better that I use the excess power than 'sell' to the grid for 4c/KW hr and then Buy back at 24c). The intention was then to time shift my grid demand to 'off-peak' at 14c/KW Hr. The worked semi OK, with a few teething problems , but eventually settled down to a grid demand of about 25% of previous grid usage. I've now installed 2KW hr solar panels to supplement battery charging and grid feed in. The is done via a Victron MPPT 150-100 which is linked to the system via CANbus.

A couple of twists to the system are

1. By using the Multiplus as a virtual 240 AC supply, the solar system can be run completely off-grid via a grid/off/generator changeover switch. The Mutliplus is the virtual generator, thus keeping the grid connect solar 'alive' during daylight hours.

2. The Multiplus is connected to the grid in offpeak times via a contactor controlled by programming the ESS to 'switch-on' the grid and charge the batteries if required. (the possibility of grid feed back is protected by use of a ZIEHL - UFR1001E)

3. When the batteries are full and there is a surplus of solar, the Multiplus Assistants are programmed eg batteries at >99% to 'turn-on' the grid and feed-in. (using relay K1). If the batteries fall to less than 99% the relay is disconnected. (unfortunately, a value of 100% can't be used (it stays 'red' when input , as logically this would give 'finer' control - this appears to be a software design issue) The grid connect switch on or Off using the relay is in parallel to the ESS option and as such provides a Boolean and/or for the ON condition.

Now to the problem I'm having.

What I'm finding is the when the batteries are near fully charged, the MPPT effectively 'turns off' by switching to either absorption or float. ie., the MPPT output is reduce to a few watts - even on a bright sunny day-. The grid connect invertor though continues to output - and feed in - directly via the Multi. at a rate of all the available system surplus (I have noted at up to 1.6 KW on some days). According to the CC screen the battery voltage is 53.8 V (This a pretty close to the actual voltage measured using a Fluke Multi-meter) which is less than the recommend maximum float voltage for the BYD packs, so it should continue to charge.

But, at the same time, my MPPT system is NOT contributing to the Feed-in.

I have experimented with increasing the voltage out settings on the MPPT, but this make little or no difference. Even if I remove the CANbus connection to the MPPT, the MPPT output does not change. I have powered down the system and restarted, without the CANbus connected to the MPPT, and again no difference.

Is there any way to program (fool) the MPPT into continuing to provide full grid feed-in when the batteries are full.

Any clues or assistance would be appreciated.


MPPT Controllers
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1 Answer
boekel avatar image
boekel answered ·

It would help if you post the settings u use, now it's all assumptions to answer.

Do you use DVCC?

do you have 'feed in excess solar power' checked?

why do you only connect at 99% SOC? the battery cannot absorb much power when full and has probably instructed the system to reduce charge current, and the system will reduce PV power from AC and DC sources. Also, at what SOC do you open the connection again?

how did you configure the assistants to 'ignore AC-input'? (screenshot of summary)

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