2 x interconnected Standalone Power Sytems with 1 acting as shore power/generator back for the other

Q&A and troubleshooting
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1

Hi There,

Got an interesting one for you all regarding the Earthing between 2 x Standalone Power Systems.

I am a licensed electrician in Australia and live on a very remote Off Grid homestead.

We have our original Standalone Power System (SPS) on our house that uses a Multiplus 24/5000 and this feeds our standard Switchboard, complete with circuit breakers, safety switches and a standard Earth rod.

This system fed our shed which is located 200m away. The shed supply is an underground feed of 4mm2 twin and Earth that was installed by a prior owner.

I know this size cable is not ideal for that run and intend to upgrade it, but for the short term I am able to use 10A @ 230V at the shed without the Voltage dropping and would like to utilize this underground cable if possible in the short term until the upgraded underground is completed.

Over time, our power needs grew so I’ve completed a second SPS at the Shed with its own switchboard, circuit breakers, safety switches and crucially, its own Earth rod.

The Shed has far better solar orientation/exposure plus much more room to increase PV capacity.

The Shed SPS uses a Multiplus 2 48/5000 GX.

My intention is to use the 4mm2 underground feed in reverse to send power back up to the House SPS from the Shed SPS.

The idea is to eliminate the need for a generator at the House SPS and have an auto start generator at the Shed.

The problem I see happening is that the 2 x earth rods (original at House and new one at Shed) might/will create earth loops and therefore nuisance tripping at both the House and Shed, plus possible dangerous currents between the 2 Earth rods .

This is because if I were to connect the Shed SPS up to the House Multiplus AC IN the single earth terminal on the Multiplus creates a permanent point of connection between the 2 earth rods.

Theoretically, I could potentially disconnect the Earth rod at the house (and deactivate the Multiplus Earth relay), but the Earth Fault Loop impedance over that distance in the underground feed would be far to high to properly activate the safety switches at the House by using the Shed Earth Rod.

Similarly, if I disconnect the Earth rod at the Shed (and the Multiplus 2 Earth relay), the Shed safety switches will not operate properly due to the high Earth Fault Loop impedance using the House Earth Rod.

So my question is this…

Using the existing underground feed, how can I have the Shed SPS feed the House SPS to act as Shore Power/Generator for the House SPS?

How can I do this and still maintain low Earth Fault Loop impedance at both the Shed SPS and the House SPS?

Or, is the only proper fix, to run a heavy duty underground Earth cable between the House SPS and the Shed SPS thereby eliminating the need for 2 Earth Rods

Thanks in advance for any advice or ideas.

2

Follow up information…

The House SPS and the Shed SPS both work perfectly fine in isolation.

On average, we use the existing generator every week or 2 on the House SPS.

The only time this Earthing issue will become apparent is if we were to try to charge the House SPS using the Shed SPS.

Essentially, by using the Shed SPS as Shore Power for the House SPS we would be creating an intermittent Earth Fault between the two systems.

Thanks again

3

First of all, feeding one inverters output to another one as the grid is a cascade, and cascading is already outside of the intended use case.

Apart from that i struggle to see where the earth loop current would come from. Im not australian, so your code might differ. But for me there can only be one single connection between neutral and earth.

In your example, if the house has no “grid”, then the internal earthing relay of the house MP will create that bond. If the “grid” is connected to the house, then that bond will be created in the sheds MP. If the generator is connected to the sheds MP, then that bond is created in the generator. To me, theres no current flowing through the earthing point, except in a fault condition.

If you have two points of connection between neutral and earth, one in the house and one in the shed, then the return current on neutral will also flow over the earth conductor as well as the actual ground between house and shed.

Now having the neutral bond 200m of 4mm2 away increases the “grid” impedance and thus lowers the potential short circuit current, maybe to a point that circuit breakers dont trip anymore.

The best thing would be to keep both systems isolated.

Next best would be to use the shed as the sole source of AC, using the MPs in parallel config as intended.

You could get away with an isolation transformer when keeping the current setup, this would let you keep both neutral-earth bonds, but doesnt solve the cascading.

Ive used a stepup transformer in the past, bringing 3x400V up to 3x950V to transmit to a neighboring building over 400m, then back down again to 3x400V. Mostly because the cable was existing and the transmit power needed increasing