How do hybrid inverters resync to grid?

[fafrd]

Excuse the newbie-to-hybrids question, but I’m hoping someone knows the answer:

Following a power outage when the UPS capability of the hybrid has taken over supplying 120V/240V split-phase power out the AC input and now grid power returns, what physically/electrically happens as the output of the hybrid has to resync to the phase f the grid?

To be more explicit about the steps and my question:

1) Grid Up Hybrid in pass-through - AC put in sync with AC input (through internal transfer switches).

2) Grid goes down (power outage) - transfer switches open as inverter takes over continuing to deliver in-sync split-phase power out AC output (while now isolated from AC input).

3) Sync list during extended power outage - after enough time running independently of (still off) grid, phase of inverter will shift and it will slide into being out of sync with the grid signal / phase.

4) Grid comes back up - reconnect to grid power. When the grid finally comes back up after an extended power outage, the hybrid will recluse the interval transfer switches and will again short AC input to AC output. This is what I am asking about.

For the sake of argument, let’s imagine that the grid signal / phase is 90-degrees or even 180-degrees out of phase with the inverter phase by the time the transfer switches close and AC input carrying grid signal is shorted to AC output.

What happens?

Is there just an abrupt one-time 90-degree of 180-degree phase shift at the AC output and if so, is that harmful to any loads such as refrigerators or any other type?

I’m trying to understand what happens at the AC-output as a hybrid inverter re-syncs to grid after an outage and whether that process can result in any excessive stress or damage to any loads being powered by the hybrid during the outage…

 

[sunshine_eggo]

Layman's babblings:

In bypass mode, they're really not synched. The AC input is shorted to the AC output, and the unit draws from input if needed to charge battery.

Transfer switch activation always involves a brief power outage on the order of 20ms. Less for high quality UPS type equipment and more for cheaper stuff. Effectively, the power is always interrupted, so you end one power source and start another.

For motors, it's typically not a big deal as their rotational inertia keeps them spinning, and they just resume driving according to the new power input. Most electronics has some charge reserve stored in capacitors to survive the interruption.

An exception is something like the Victron with its powerassist. In bypass, it's no different, but the inverter can actually sync to the signal and supplement it analogous to a grid tie inverter to boost the output. Hybrids may have a similar capability as well.

 

[fafrd]

Layman's babblings:

In bypass mode, they're really not synched. The AC input is shorted to the AC output, and the unit draws from input if needed to charge battery.

Transfer switch activation always involves a brief power outage on the order of 20ms. Less for high quality UPS type equipment and more for cheaper stuff. Effectively, the power is always interrupted, so you end one power source and start another.

For motors, it's typically not a big deal as their rotational inertia keeps them spinning, and they just resume driving according to the new power input. Most electronics has some charge reserve stored in capacitors to survive the interruption.

An exception is something like the Victron with its powerassist. In bypass, it's no different, but the inverter can actually sync to the signal and supplement it analogous to a grid tie inverter to boost the output. Hybrids may have a similar capability as well.

Thanks for the layman’s explanation .

So would you have any concern about wear on ridges or any other loads by being powered by a hybrid which is cycled off and back on grid once per day?

Many of the newer hybrids offer a prioritization function whereby you can prioritize loads to be powered by incoming DC-coupled solar power first then grid power before inverting battery power as well, so I assume that would require similar capability to what Victron uses to synchronize inverter with grid for Power Assist, right?

 

[sunshine_eggo]

Thanks for the layman’s explanation .

Too much credit... vomiting my very limited understanding...

So would you have any concern about wear on ridges or any other loads by being powered by a hybrid which is cycled off and back on grid once per day?

Probably not.

Many of the newer hybrids offer a prioritization function whereby you can prioritize loads to be powered by incoming DC-coupled solar power first then grid power before inverting battery power as well, so I assume that would require similar capability to what Victron uses to synchronize inverter with grid for Power Assist, right?

With a function like that, it sounds an awful lot like grid-tie/powerassist, so yeah.

 

[Hedges]

Sunny Island is what I know.

After it sees stable AC on input (for 5 minutes), it adjusts its frequency a bit off 60 Hz so phase slips into alignment. While maintaining matching phase, it closes relay to connect grid (or generator) input.

I don't know whether it also synchronizes voltage. It could match Vrms, although distorted waveforms wouldn't be exactly the same.

If it has AC coupled sources like PV being curtailed by frequency-watts, it first raises frequency until they are knocked offline, then synchronizes.

 

[RCinFLA]

Initially, when AC input is first applied, the pass-through relay must be open.

The inverter detects input AC voltage and begins to realign its freq/phase/voltage to match that of AC input. This process takes 10 seconds to a minute depending on how much difference there is in frequency between initial AC input and inverter frequency. The inverter will not abruptly change its frequency or phase as certain AC loads, like AC motor, can create a large surge current (forward or backward) if phase is changed too quickly. It must slew alignment to AC input slowly.

This is why inverter input AC frequency limits for grid AC input is usually set tighter than a generator AC input since it is not necessary for inverter to search outside of grid specs frequency range as it would just possibly extend the amount of time to acquire lock on AC input.

Once inverter sync's up to AC input the pass-through relay is closed and inverter runs in parallel with AC input.

Normal sync is maintained on AC input by a zero-crossing phase detector that makes small inverter freq increments up and down depending on if AC input or inverter phase timing crosses zero first. It must be small enough and slow enough to ride through AC glitches caused by turn on/off surge loads.

For a synchronous, constant rpm generator, this also means the generator's governor rpm control must be smooth and steady in engine rpm's or there will be freq/phase shifts in generator output that will cause inverter not to maintain sync and cause it to release pass-through relay connection to generator.

The most important thing to realize is when pass-through is open, the inverter freq/voltage is internally determined (inverter is its own master). When AC input pass-through relay is closed, the inverter is slaved to follow AC input.

Disconnect from AC input is a bit more complicated. If grid drops out, there is a momentary overload on inverter as inverter tries to power the collapsed grid, up to possibly the surge current limit of inverter, before it releases AC input pass-through relay. Pass-through relay contacts take a beating when operating on grid when there is three to five disconnect grid induced glitches per month with inverter operating in parallel UPS mode.

Toughest thing for inverter to do is detecting when AC input goes open circuit, like when you open AC input breaker. Zero AC input current is a legit condition and voltage on AC input does no good since inverter is feeding through pass-through relay to AC input.

Most inverters normally use the up/down pulses from phase detector, to determine when to release pass-through relay. When a legit AC input is present there should be a repetitive average of up and down phase correction pulses. Some inverters, when AC input is opened, just wander off in frequency until they hit max inverter run AC frequency limit before disconnecting pass-through relay. After disconnecting pass-through relay to AC input the inverter slowly pulls frequency and voltage back to its self-mastering center point.

Recognition of loss of AC input can take from a couple of seconds to a minute before it releases pass-through relay. It is very important that AC input is not reapplied while the pass-through relay is still engaged. This is a big problem when an immediate crossover ATS switch is used between a generator and grid inputs. If the AC input is significantly out of phase with inverter, a large surge current will result. It can pop input breaker, damage pass-through relay contacts, or even damage inverter. When the AC input and inverter are fully out of phase it is much worse than a grid collapse for surge current.

 

[fafrd]

Initially, when AC input is first applied, the pass-through relay must be open.

The inverter detects input AC voltage and begins to realign its freq/phase/voltage to match that of AC input. This process takes 10 seconds to a minute depending on how much difference there is in frequency between initial AC input and inverter frequency. The inverter will not abruptly change its frequency or phase as certain AC loads, like AC motor, can create a large surge current (forward or backward) if phase is changed too quickly. It must slew alignment to AC input slowly.

This is why inverter input AC frequency limits for grid AC input is usually set tighter than a generator AC input since it is not necessary for inverter to search outside of grid specs frequency range as it would just possibly extend the amount of time to acquire lock on AC input.

Once inverter sync's up to AC input the pass-through relay is closed and inverter runs in parallel with AC input.

Normal sync is maintained on AC input by a zero-crossing phase detector that makes small inverter freq increments up and down depending on if AC input or inverter phase timing crosses zero first. It must be small enough and slow enough to ride through AC glitches caused by turn on/off surge loads.

The most important thing to realize is when pass-through is open, the inverter freq/voltage is internally determined (inverter is its own master). When AC input pass-through relay is closed, the inverter is slaved to follow AC input.

Disconnect from AC input is a bit more complicated. If grid drops out, there is a momentary overload on inverter as inverter tries to power the collapsed grid, up to possibly the surge current limit of inverter, before it releases AC input pass-through relay.

Toughest thing for inverter to do is detecting when AC input goes open circuit, like when you open AC input breaker. Zero AC input current is a legit condition and voltage on AC input does no good since inverter is feeding through pass-through relay to AC input.

Most inverters normally use the up/down pulses from phase detector, to determine when to release pass-through relay. When a legit AC input is present there should be a repetitive average of up and down phase correction pulses. Some inverters, when AC input is opened, just wander off in frequency until they hit max inverter run AC frequency limit before disconnecting pass-through relay. After disconnecting pass-through relay to AC input the inverter slowly pulls frequency and voltage back to its self-mastering center point.

Recognition of loss of AC input can take from a couple of seconds to a minute before it releases pass-through relay. It is very important that AC input is not reapplied while the pass-through relay is still engaged. This is a big problem when an immediate crossover ATS switch is used between a generator and grid inputs. If the AC input is significantly out of phase with inverter, a large surge current will result. It can pop input breaker, damage pass-through relay contacts, or even damage inverter. When the AC input and inverter are fully out of phase it is much worse than a grid collapse for surge current.

Thank you for the wonderful explanation - more detail than I could have hoped for.

I’m mulling using a contactor on AC input to take a hybrid off grid at sundown and then take it back on grid when battery is depleted or solar generation begins in the morning (whichever occurs first).

From what you have described, it does not seem that using a hybrid inverter in this manner should pose a problem either to the inverter or to the equipment including motors powered by it through the AC input.

From your description, I certainly understand the importance of waiting a suitable length of time before reconnecting or re-disconnecting grid from AC input…

 

[RCinFLA]

Thank you for the wonderful explanation - more detail than I could have hoped for.

I’m mulling using a contactor on AC input to take a hybrid off grid at sundown and then take it back on grid when battery is depleted or solar generation begins in the morning (whichever occurs first).

From what you have described, it does not seem that using a hybrid inverter in this manner should pose a problem either to the inverter or to the equipment including motors powered by it through the AC input.

From your description, I certainly understand the importance of waiting a suitable length of time before reconnecting or re-disconnecting grid from AC input…

The time should be more than long enough for pass-through relay to release when AC input is removed.

Before you go to extra trouble check the features in inverter first. Some have the ability to setup user parameters to do what you want to do without any external added grid relay disconnect. The internal pass-through relay can perform the same function with the right inverter features.

Adding external AC input relays or ATS switches is more likely to get you in trouble with closed pass-through relay and out of sync AC input phase.

The reason why some inverters have separate AC inputs (AC1 and AC2) for grid and generator, is the inverter can internally manage the resync'g to another AC input phase source. It uses inverter-battery power to supply power during the time it releases from generator to resync and reconnect to grid AC input.

 

[fafrd]

The time should be more than long enough for pass-through relay to release when AC input is removed.

Before you go to extra trouble check the features in inverter first. Some have the ability to setup user parameters to do what you want to do without any external added grid relay disconnect. The internal pass-through relay can perform the same function with the right inverter features.

Victron, yes, but not the budget AIOs I’m considering…

https://www.srnesolar.com/wp-content/uploads/2022/12/SRNE_ASF-series_48V_8-10kW_split-phase_solar-charger-inverter_usermanual_1.4.pdf