G100 export limiting

[Oldgreybeard]

Have used two of those for over a year. They work well.

Many thanks, that's very useful feedback.

The simplest way to do this seems to be to fit another current transformer to the big PV system AC output, and use that to switch a double pole contactor to isolate the second PV system from the grid. Seems this may be easier than I thought, as I've just found these cheap current sensors: https://www.ebay.co.uk/itm/304631567636 They are adjustable from 2A to 30A, and seem ideal, as I can set it to about 26A and it will trigger a contactor directly.

These are self-powered, from the current flowing through the sensed wire, so can safely be tucked into the consumer unit, with the line conductor from both PV inverters running through the hole.

If you're sensing the combined current and using that to disable the 2nd system won't the relay keep clicking in and out? If you're just sensing from the 1st system and using that to switch off the 2nd system then no problem.

Good point, thanks, you're right, without some sort of time delay relay this could get into a state where the contactor keeps cycling on and off. I need to sense the combined current, though, as the DNO concern is with the export exceeding the 6kW rating of the big system inverter. It's just about possible that the main system could be generating over 4kW at a time when the second system is generating its 2kW maximum.

I think all it needs is a timer relay, so the contactor is switched off when the 6kW threshold is reached, then stays off for, say, 30 minutes. Because of the difference in orientation between the two PV arrays it's pretty unlikely that the 6kW limit will be reached that often, and only then for a short time. Just adding a delay before the second system can turn back on when the output drops should be enough to stop the system "short cycling". Something like this: https://www.ebay.co.uk/itm/134250349231 configured with an "off delay", so when triggered it stays on for the set period, then turns off and allows the second PV system to reconnect.



My gut feeling is that this limiting system may not ever operate, anyway, given that the second system will be in shade by the time the big system peaks in the early afternoon. I only really need this as a way to keep the DNO happy and tick a box in their records to say that the export is always limited to no more than 6kW.

[ALAN/ALAN D]

With the Current transformers I have been using the relay contacts to switch solid state relays On/Off
So relay contacts passing very small currents. The solid state relay could supply power to the contactor coil.
I still can not understand how a multifunction timer can be produced for £20
Many years ago they would have cost close to £100

[Oldgreybeard]

With the Current transformers I have been using the relay contacts to switch solid state relays On/Off
So relay contacts passing very small currents. The solid state relay could supply power to the contactor coil.
I still can not understand how a multifunction timer can be produced for £20
Many years ago they would have cost close to £100

It is staggering just how cheap things have got. This morning I received half a dozen Seeed Studio Xiao ESP32C3 boards: https://www.seeedstudio.com/Seeed-XIAO- ... -5431.html

$5 each and they contain a complete WiFi subsystem, lithium battery charger and BMS and the ability to interface to a wide range of sensors. Within half an hour I had one hooked up to a single lithium cell, an SHT30 temperature/humidity sensor and sending data back to another ESP32 that acts as an ESP-NOW bridge to Home Assistant. Quite amazing, as it will run for well over a year on that single small lithium cell. If I add a tiny solar panel then the thing would almost run indefinitely if it's anywhere that gets a bit of light. Means I can put sensors all over the place and have them send data to Home Assistant very easily, just a matter of working out what's worth measuring.

[Caesium]

It is staggering just how cheap things have got. This morning I received half a dozen Seeed Studio Xiao ESP32C3 boards: https://www.seeedstudio.com/Seeed-XIAO- ... -5431.html

Ooh they look nice.. ordered 5

I've used quite a few of the older model, esp8266, in little projects for temperature monitoring and suchlike, but not tried the newer esp32. I like the built in bluetooth, is your temperature sensor using that to send to the master esp32? Presumably it must go into deep sleep a lot to last a year on a single small cell.

[Oldgreybeard]

It is staggering just how cheap things have got. This morning I received half a dozen Seeed Studio Xiao ESP32C3 boards: https://www.seeedstudio.com/Seeed-XIAO- ... -5431.html

Ooh they look nice.. ordered 5

I've used quite a few of the older model, esp8266, in little projects for temperature monitoring and suchlike, but not tried the newer esp32. I like the built in bluetooth, is your temperature sensor using that to send to the master esp32? Presumably it must go into deep sleep a lot to last a year on a single small cell.

I'm using the ESP-NOW protocol to send data. This works by knowing the MAC address of every connected device and allows unidirectional, bidirectional, one-to-one, or one-to-many networks. Big advantage is that the transmitters don't need to stay on, as they do with normal WiFi. Devices can wake up, read sensors and transmit data, then go back to sleep again. This is key to the energy efficiency. If interfacing to something like Home Assistant you do need an ESP32/ESP8266 to act as a bridge from the ESP-NOW side to either something like MQTT or perhaps a hard wired connection so that Home Assistant can pull (or push) data to the ESP-NOW network. Only real limitation with ESP-NOW is that data transmissions cannot exceed 250 bytes in one go, which isn't a practical restriction at all.

[sharpener]

The simplest way to do this seems to be to fit another current transformer to the big PV system AC output, and use that to switch a double pole contactor to isolate the second PV system from the grid. Seems this may be easier than I thought, as I've just found these cheap current sensors: https://www.ebay.co.uk/itm/304631567636 They are adjustable from 2A to 30A, and seem ideal, as I can set it to about 26A and it will trigger a contactor directly.

These are self-powered, from the current flowing through the sensed wire, so can safely be tucked into the consumer unit, with the line conductor from both PV inverters running through the hole.

If you're sensing the combined current and using that to disable the 2nd system won't the relay keep clicking in and out? If you're just sensing from the 1st system and using that to switch off the 2nd system then no problem.

Good point, thanks, you're right, without some sort of time delay relay this could get into a state where the contactor keeps cycling on and off. I need to sense the combined current, though, as the DNO concern is with the export exceeding the 6kW rating of the big system inverter. It's just about possible that the main system could be generating over 4kW at a time when the second system is generating its 2kW maximum.

I think all it needs is a timer relay, so the contactor is switched off when the 6kW threshold is reached, then stays off for, say, 30 minutes.

Or you could add hysteresis in some way. E.g. when the threshold is reached not only does the relay drop out the second PV source but also switches to using a second CT with a lower setting so it doesn't immediately come back on. In a similar situation this is how the Eddi can be programmed AFAIR.

[Oldgreybeard]

Or you could add hysteresis in some way. E.g. when the threshold is reached not only does the relay drop out the second PV source but also switches to using a second CT with a lower setting so it doesn't immediately come back on. In a similar situation this is how the Eddi can be programmed AFAIR.

Certainly more elegant to add hysteresis than just use a time delay relay. Probably not much difference in cost, either, as these current sense switches are so cheap. Just needs a bit of relay logic to get the main contactor to switch the inverter off at the higher threshold and back on again at the lower threshold. It does increase the relay count, though.