Grid vs Solar AC power sources - Phasing of the AC Voltage is why we must switch between them... Right?

TedH

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I saw a post on another forum where somebody asked if they could add another AC voltage source while grid connected. I wanted to say there would be issues. After consideration, I realized the issue would be the phase of the two power sources; they would not be in phase. Therefore, if I had a power source, say from an inverter producing AC voltage from inputs from Solar Charge Controller and Lifepo4 batteries, I would not want to introduce that power when Grid power is live; I would need to switch between the two sources. If I tried to introduce the inverter AC voltage, I would burn the inverter and risk fire or electricution. It is all due to the phasing of the alternating current... correct? That is why my inverter has an Automatic Transfer Switch integrated. If it senses steady Grid power, it is set to select the Grid power. If Grid power is interrupted, it will switch to the DC battery/solar power source and convert it to Alternating current to serve the load. The inverter chooses between the AC voltage from GRID and producing from inverter AC voltage.
 

RCinFLA

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PV fed Grid tied inverters, hybrid inverters, and most UPS backup supplies do this. For GT inverters there are also other regulations for shutdown time in the case where grid goes down so they do not continue to operate pushing power to an open grid. For a hybrid grid tie inverters and UPS's they may have drifted off original grid phasing during backup operation so when grid reappears there is a few seconds where they slowly resynchronize to grid before switching back over to grid supply for their loads.

Just an A-B transfer switch, for something like a backup generator, does not. It usually waits several seconds to make the change over so things like AC motors shut down completely before getting random phase AC from generator reapplied. In the case of a backup generator you also have the startup and stabilizing time so it generally means no power for a minute or two. UPS and hybrid inverters are already in standby with their timing pre-synchronized to grid, waiting for grid failure so they can almost immediately switch over in less then one AC cycle.

A hybrid inverter can suppliment incoming AC power. For example, if AC output loads exceeds a generator power capability the batteries through inverter can add to the generator power to supply the greater output load.
 
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TedH

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What I am asking is to validate the reasoning to NOT introduce AC power from inverter while the GRID is powering. I have the ATS and love how it works in my 6000watt/18000watt surge Aims pure sine wave inverter/charger. I'm assuming that we can't just introduce the AC (or remove for that matter) WHILE the Grid power is flowing. If even for a nanosecond, there needs to be an interrupt to prevent fire/failure and to protect our appliances on load side. Correct?
 

Bud Martin

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Grid-tie inverter has synchronizer circuit to allow the inverter output to be in sync with utility line and also to shutdown the inverter if the utility is not present, regular INVERTER does not have that function.
 

TedH

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Grid-tie inverter has synchronizer circuit to allow the inverter output to be in sync with utility line and also to shutdown the inverter if the utility is not present, regular INVERTER does not have that function.
What is the difference between grid-tied inverters (I think I know) and RV inverter/chargers that connect to Grid? Are Grid-tied inverters specialized such that they are integrated into a fixed grid/solar/storage solution and ONLY feed the grid with solar power?

I am assuming you cannot live without the grid since the grid-tie system harvests solar and sells it to the local utility vs. being a solution that supports load in event Grid is down (characteristic of an RV system that can be grid-connected or on solar or other power sources.
 

RCinFLA

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What is the difference between grid-tied inverters (I think I know) and RV inverter/chargers that connect to Grid? Are Grid-tied inverters specialized such that they are integrated into a fixed grid/solar/storage solution and ONLY feed the grid with solar power?

I am assuming you cannot live without the grid since the grid-tie system harvests solar and sells it to the local utility vs. being a solution that supports load in event Grid is down (characteristic of an RV system that can be grid-connected or on solar or other power sources.
PV grid tied inverters do not have batteries. No backup power when grid goes down, Hybrid grid tied inverter have a battery to power inverter that takes over supplying output AC power when grid goes down.

A hybrid inverter does not need an external A-B transfer switch. It only has an internal AC input relay to parallel the inverter to grid when they are in sync The inverter is permanently connected to AC output. The current rating of input relay contacts limits the maximum current pass through allowed from AC grid or generator at AC input. It is usually higher amperage than inverter alone.

This is the same principle used by power utilities to combine multiple power generators feeding the grid. They have to spin up an alternator until it matches phase of grid before they close the connection to grid to bring additional generators online.
 

TedH

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PV grid tied inverters do not have batteries. No backup power when grid goes down, Hybrid grid tied inverter have a battery to power inverter that takes over supplying output AC power when grid goes down.

A hybrid inverter does not need an external A-B transfer switch. It only has an internal AC input relay to parallel the inverter to grid when they are in sync The inverter is permanently connected to AC output. The current rating of input relay contacts limits the maximum current pass through allowed from AC grid or generator at AC input. It is usually higher amperage than inverter alone.

This is the same principle used by power utilities to combine multiple power generators feeding the grid. They have to spin up an alternator until it matches phase of grid before they close the connection to grid to bring additional generators online.
Good info. Thanks!
 
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