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3P Mains to Single Phase Inverter Changeover


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Hi, I want to automatically switch from 3 phase mains supply to single phase inverter output during grid loss events using a pair of interlocking contactors as shown in the diagram below.

Contactor coils (A1/A2) and auxiliary contacts (NC) will be wired in a similar fashion to this diagram but with mains supply as the priority source. Shedding of high power or 3P loads will be done downstream using high precision under/over voltage relays that will differentiate between 240-250VAC mains supply and 228-230VAC inverter output.

Given that typical contactor transfer times are 10-20ms, will this setup work without shorting the phases?   

 

3P Mains to 1P Inverter Changeover.png

Edited by Kilowatt Power
Grammar
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Unfortunately this scheme will not work if you have 3-phase loads (e.g. motors).  The problem is that each of the incoming phases of the 3-phase mains are 120 Degrees out-of-phase with each other - there is approximately 400V between each phase.  To suddenly switch this to single phase, with 0V difference between the phases, will cause havoc with any 3-phase loads!

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2 hours ago, Richard Mackay said:

There'es no need to switch the neutrals IMHO

I thought so too. Moreover a 3 pole contactor of the same rating is also much cheaper.  However I read somewhere that some inverters like Infinisolar will throw an error if the AC input neutral is directly connected to the inverter output neutral. A 4 pole contactor prevents such an occurrence.

2 hours ago, Richard Mackay said:

How fast does the switchover need to be?

Fast open transition (break before make) with NO possibility of shorting the phases. I believe 10 - 20ms transfer time will provide sufficient isolation.

2 hours ago, Richard Mackay said:

Methinks the logic challenges of priority, day/night etc. will ultimately require a controller (PLC)

It's a simple battery power backup solution. Switching from grid to inverter during power outages and back when power is restored. 

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1 hour ago, NigelL said:

Unfortunately this scheme will not work if you have 3-phase loads (e.g. motors). 

Definitely. There's a 3 phase pool pump which will be isolated downstream using a high precision under/over voltage relay & contactor i.e. contactor disengages if phase to neutral voltage is less than 230VAC (inverter power).

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1 hour ago, Kilowatt Power said:

It's a simple battery power backup solution. Switching from grid to inverter during power outages and back when power is restored. 

What loads do you have connected? (and which of these are you needing to keep powered during power outages?)

 

Edited by Richard Mackay
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You may still run into problems if there are single-phase loads that suddenly get a step-change in voltage due to the sudden phase change, as the contactors switch over. It may also be tricky to coordinate the disconnection of 3-phase loads and the switching of the contactor.  There will be a lot of smoke and tears if there is even the briefest overlap 😬 

A single-phase Inverter must synchronise its output to one of the three incoming mains phases - and should then only supply power to loads that same single phase. This is normally done via a change-over relay that cannot ever allow a cross-connection between the inverter and the incoming mains, even with a relay failure.

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3 hours ago, NigelL said:

You may still run into problems if there are single-phase loads that suddenly get a step-change in voltage due to the sudden phase change, as the contactors switch over. It may also be tricky to coordinate the disconnection of 3-phase loads and the switching of the contactor.  There will be a lot of smoke and tears if there is even the briefest overlap 😬 

The changeover contactor is mechanically (and electrically) interlocked. It is not possible to have the two contactors energised at the same time.

These contactors are used in reversing drives when a phase is reversed to change direction of a 3 phase motor.

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4 hours ago, Richard Mackay said:

What loads do you have connected? (and which of these are you needing to keep powered during power outages?)

Typical loads found in a townhouse. The house has 3 consumer units with no conduit runs between them. A load-shedding contactor in each CU will isolate high power loads i.e. sockets for kitchen appliances, cooker, oven, under-sink water heater and solar water heater booster elements whenever the voltage goes below 230VAC. Lights and all other receptacles will remain powered during outages.

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5 hours ago, NigelL said:

You may still run into problems if there are single-phase loads that suddenly get a step-change in voltage due to the sudden phase change, as the contactors switch over.

Thus begs the question, will a 10ms gap between switching of contactors provide sufficient cushion against phase interaction?

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12 hours ago, Kilowatt Power said:

 

ATS Detailed Wiring.png

Is there a specific reason why you switch neutral in the control circuit (solar auxiliary)? normally only the live should be switched.

 

1 hour ago, Kilowatt Power said:

will a 10ms gap between switching of contactors provide sufficient cushion against phase interaction?

In my own system I have only single phase that switches between solar and grid, probably also about 10ms and have no issues. I switch even big loads like fridge, washing machine, tumble drier etc. I can hear the motor ramping up or down on the vacuum cleaner or see light dim or bright as my grid voltage is 245V and my solar 220V but I had no problems yet. I doubt your system will have any problems switching to single phase as long as there is no single phasing of 3 phase motors. 

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19 minutes ago, Gerrie said:

In my own system I have only single phase that switches between solar and grid, probably also about 10ms and have no issues. I switch even big loads like fridge, washing machine, tumble drier etc. I can hear the motor ramping up or down on the vacuum cleaner or see light dim or bright as my grid voltage is 245V and my solar 220V but I had no problems yet. I doubt your system will have any problems switching to single phase as long as there is no single phasing of 3 phase motors. 

Good point about the 3 phase motors single phasing. I would install a 3 phase protection relay for each motor.

This would be more solid than the under/over voltage relays that were mentioned.

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2 hours ago, Gerrie said:

Is there a specific reason why you switch neutral in the control circuit (solar auxiliary)? normally only the live should be switched.

I got the circuit off the Internet and hadn't realised that. Thanks. Shall swap the solar auxiliary contact leads at the mains input side for electrical interlocking.

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6 hours ago, Kilowatt Power said:

I got the circuit off the Internet and hadn't realised that. Thanks. Shall swap the solar auxiliary contact leads at the mains input side for electrical interlocking.

If I’m correct the Italians switch neutrals on their control circuits, but in S.A. we use a neutral as a common throughout. It also makes faultfinding a bit easier🙂

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A big advantage of having the neutrals common is keeping the different voltages tied together.

If you don't do this they can float and have a big voltage difference between them.

If you have used a DC power supply (e.g. for control voltage) you need to connect all negative connections together (and ensure a loose connection to earth as well)

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