Emergency Power option - Automatic Switch

[Stinsy]

Which inverter do you have? As I said the Solis one I have has a "back up" supply. If you enable it and connect it then it "passes" power from the grid straight through to the back up socket. If the grid fails then it automatically switches to power it from the PV/batteries via the inverter, apparently within 20ms -effectively like a UPS.

I understand about it being "automatic" so that other members of the family don't have to worry if you are away. I have the same issues here but decided that there were too many variables to explain - I just make sure my wife knows where a torch is and the phone number to report a power cut. In spite of the number of power cuts we get the situation has never arisen yet.

There are two different ways inverters handle this:

UPS: where the inverter has an output that is always on, even in the event of an outage…

EPS: where the inverter has an output that is only on in the event of an outage, these need complex changeover switches…

[cojmh]

Which inverter do you have? As I said the Solis one I have has a "back up" supply. If you enable it and connect it then it "passes" power from the grid straight through to the back up socket. If the grid fails then it automatically switches to power it from the PV/batteries via the inverter, apparently within 20ms -effectively like a UPS.

I understand about it being "automatic" so that other members of the family don't have to worry if you are away. I have the same issues here but decided that there were too many variables to explain - I just make sure my wife knows where a torch is and the phone number to report a power cut. In spite of the number of power cuts we get the situation has never arisen yet.

I have a Solax SKU5000E

Thanks

[rogeriko]

I have the same passthrough inverter.
The mains is on a simple digital timer that operates a high current relay/contactor. This operates at 12.30 to 4.30 so I can charge my battaeries at 7p kwh electric. The output runs 4 large freezers and all the lights in the house and gaeden. It has been running non stop for a year now. If we ever have a power cut we will never know unless the sockets stop working.

[rogeriko]

Inverter is 5kw. £408.

No need for any disconnect switches or notifing the DNO because it does not feed into the grid.

I can and will add solar panels to it because there is an MPPT solar input for battery charging. 48v.

[cojmh]

Inverter is 5kw. £408.

No need for any disconnect switches or notifing the DNO because it does not feed into the grid.

I can and will add solar panels to it because there is an MPPT solar input for battery charging. 48v.

Thank you for the information.

Whilst the EPS is not a flawless change over (I think there is a couple of seconds switch over) I am hoping to get to the point that we wouldn't notice too much the switch over.

[marshman]

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[cojmh]

Be aware that the majority of compressors on fridges and freezers do not like the power being switched off and then back on again in a short time if they are running. They effectively stall and draw a high current for a good few seconds before tripping off. They will restart later on but the power draw whilst stalled is quite high. I had this several times in the early days of the change over switch and it was a while before I worked out what was happening. I believe the reason they won't start is that if they were running then they have built up pressure and the starting torque cannot overcome the residual pressure in the system.

There are some explanations here, also a graph illustrating how high the currents can be - up to 7 times normal running current - which may well upset the inverter.

Nothing is ever simple!

thank you for the heads up ... I will look into this as knowing my luck they would both go at the same time during switch over!

Cheers,

[marshman]

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[nowty]

Be aware that the majority of compressors on fridges and freezers do not like the power being switched off and then back on again in a short time if they are running. They effectively stall and draw a high current for a good few seconds before tripping off. They will restart later on but the power draw whilst stalled is quite high. I had this several times in the early days of the change over switch and it was a while before I worked out what was happening. I believe the reason they won't start is that if they were running then they have built up pressure and the starting torque cannot overcome the residual pressure in the system.

There are some explanations here, also a graph illustrating how high the currents can be - up to 7 times normal running current - which may well upset the inverter.

Nothing is ever simple!

thank you for the heads up ... I will look into this as knowing my luck they would both go at the same time during switch over!

Cheers,

When I had my older On Grid /Off Grid system and used to switch between them with a change over relay based on battery SOC, if the fridge was running when the change over relay switched, the fridge compressor always threw a wobbly, although it never broke and is still working.

My system is now changed where I have both a small UPS and a large EPS.

The small UPS is for the internet, satellite box, telephone, and computer so I never have an interruption of service.

I use my SMA Sunny Island battery as an EPS, so if the external power fails I lose the power for 2 or 3 secs whilst the Sunny Island cuts the live grid connection with a 100A power contactor and it also cuts my EV charging circuits with another 63A power contactor. These contactors are failsafe in that they are in the OFF state if un-energised.

The power then comes back on automatically to the house via the 6kW Sunny Island battery inverter using all the same wiring and the neutral is still connected to maintain a PME. The PME is also backed up with a local earth rod so even if I lose the external TNCS PME because of a cable fault in the street, the system turns into a TT earthing system.

The short outage switches off several WiFi sockets which switches off other high power circuits such as the heatpump, storage heater boosting and immersions heaters so I don't get an instant overload on the sunny Island when running in Off Grid mode.

The beauty is the solar PV inverters still reconnect and are then throttled to avoid overvoltage by the Sunny Island through frequency shift control. So in summer I could run for months with a grid outage.

When the external power comes back, the Sunny Island senses this and has to match the grid voltage, synchronise with the grid frequency and shut down the solar PV inverters if they are being throttled as they will operating at a slightly higher frequency than the grid.

Then the live grid feed and the EV charger circuit are then re-connected and shortly after the solar PV inverters reconnect. So I don't lose power again on re-connection with the grid.

[marshman]

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