Adding micro inverter system to string inverter system?

[C.R.J.]

I have a string inverter and panels at 2.5 kw. Panels are on my roof. I want to increase the system to possibly 3.5-4 kw.
The only space i have for more panels is some distance away. To increase the power of the installation i would need to upgrade my inverter and run the DC cables around 75 metres. reinforced concrete is also in the way, so not a simple task.
The roof of the building 75 metres away, does have a distribution box and associated circuit breakers for that building. It is however getting its feed from the main house distribution panel.
The main house distribution panel is where my string inverter is connected to.(Via breakers etc).
My question is, can i install panels with micro inverters on the building 75 meters away and connect to the distribution panel in the same building, without compromising the string inverter system.
The cost of using the microinverters is much less than upgrading my inverter and digging up loads of reinforced concrete.
This is purely a technical question. I know there are regulatory issues.
Personally i will say the idea of the microinverters is not something i feel entirely happy about doing. Which is why i ask the question.

[Mike90250]

Electrically, it makes no difference to put 3.5 KW of micros, or another string inverter rated for 4kw on the remote roof. This would not affect your existing gear at all.. The only issue is if the existing wiring between the remote roof @ 75 meters away, can handle the extra power. You may have to pull new AC cables to handle the extra power.
This also assumes your main panel that this ties into, is rated for the extra power.
choices:
1) micro inverters
2) an additional string inverter at the new array .

Local code may complicate things if you need Rapid Shutdown on the new roof

If the 75 meters of wiring is insufficient, as your solar approaches peak power daily, it may sense too high of a grid voltage and things will shut down for 5 min, then start up, then shut down again till the solar reduces enough to allow the existing wiring to handle the power.

[C.R.J.]

Thank you for your insight.
Indeed the rating of the cable between the two buildings will be a limiting factor. I have done some calculations and factoring in a 20% tolerance, i will be limited to 2kw extra power.
That is not as great as i would like, but we dont always get what we want. Unfortunately i cannot change the cable between the buildings without major work digging up reinforced concrete.
Fortunately the main panel is rated for the extra power. Local code is not going to affect the installation if I go ahead with the additional 2kw.
I will post an update on this.
Thanks. CRJ

[Ampster]

Thank you for your insight.
Indeed the rating of the cable between the two buildings will be a limiting factor. I have done some calculations and factoring in a 20% tolerance, i will be limited to 2kw extra power.
That is not as great as i would like, but we dont always get what we want.

Cable is rated based on Amps. The higher the voltage the more power a given current limitation can pass. There was not enough detail for me to make a recommendation but If I knew the wire size feeding the sub panel it might be easier to understand the limitation you referred to?

[C.R.J.]

My calculations were based on the cable being 1.5mm. On checking properly it turns out the cable is 2.5mm.

[mdscole]

I did exactly as you are proposing. String array on the roof of the house and an IQ7 micro inverter system on a patio cover. There was a 50 amp power on that corner of the house for a jacuzzi that never materialized. Used that feed for a sub panel and fed the micro inverters into that box. Works perfectly. Additional array was only 8 x 285w panels. due to the lower height of the patio cover vs a 2 story roof - the micros do a great job dealing with shading.

The only lesson learned: For Enphase inverters, you do need to buy the controller box. (Envoy). Those micros are programmable for all kinds of voltages, frequencies and the only way to set that up well was with the Envoy and a few hours on the tech support line with Enphase. Eventually had to install a simple electrical noise filter so that the Envoy could clearly communicate with the inverters across the power line. There are the $400 versions from Enphase - but with a little 2280W system, I got away with the $17 variety. Just make sure the entire array and envoy are on the load side of this thing.
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[ajonestx]

Thank you for your insight.
Indeed the rating of the cable between the two buildings will be a limiting factor. I have done some calculations and factoring in a 20% tolerance, i will be limited to 2kw extra power.
That is not as great as i would like, but we dont always get what we want. Unfortunately i cannot change the cable between the buildings without major work digging up reinforced concrete.
Fortunately the main panel is rated for the extra power. Local code is not going to affect the installation if I go ahead with the additional 2kw.
I will post an update on this.
Thanks. CRJ

random question. is the underground cable in conduit and could you use the existing cable as a pull string to pull in new cable of the appropriate size?

[C.R.J.]

I did exactly as you are proposing. String array on the roof of the house and an IQ7 micro inverter system on a patio cover. There was a 50 amp power on that corner of the house for a jacuzzi that never materialized. Used that feed for a sub panel and fed the micro inverters into that box. Works perfectly. Additional array was only 8 x 285w panels. due to the lower height of the patio cover vs a 2 story roof - the micros do a great job dealing with shading.

The only lesson learned: For Enphase inverters, you do need to buy the controller box. (Envoy). Those micros are programmable for all kinds of voltages, frequencies and the only way to set that up well was with the Envoy and a few hours on the tech support line with Enphase. Eventually had to install a simple electrical noise filter so that the Envoy could clearly communicate with the inverters across the power line. There are the $400 versions from Enphase - but with a little 2280W system, I got away with the $17 variety. Just make sure the entire array and envoy are on the load side of this thing.
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Thanks mdscole for the reply. That helps tremendously. I am indeed looking at using enphase inverters. The noise filter is not something I would have initially considered. That is a good saving when compared with the enphase model. Thanks.
CRJ

[C.R.J.]

random question. is the underground cable in conduit and could you use the existing cable as a pull string to pull in new cable of the appropriate size?

Sadly it is not possible. It was the first thing i tried. The original builder of the property (30 years ago), did not allow for such a thing. It seems there are numerous tight 90 degree bends along the length of the cable run, and it is also buried to a depth of over 1 metre in some places.