How does an inverter synchronise to a generator?

toms

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In an off grid system where the inverter has a generator input, can someone please explain how the inverter synchronises its power waveform to the generator. I’m curious how this happens without affecting the existing loads on the generator and inverter.
Cheers
 

RCinFLA

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You are referring to a battery equipped hybrid inverter that accepts AC input from grid or generator.

Hybrid inverter has inverter permanently connected to AC output. There is a connect relay between AC input and inverter. Relay is normally open. When an AC input voltage is detected and within acceptable voltage/frequency range the inverter starts a slow phase tracking adjusment to match the AC input phase. The inverter also is adjusted to match AC input voltage which is the easy part. Once AC phase and voltage of inverter matches AC input the input relay closes putting the inverter in parallel with AC input (grid or generator).

PWM inverter can modify its AC voltage slightly when running in parallel wth grid. If inverter puts out a little more voltage then AC input voltage the inverter pushes out power. If inverter is adjusted to out a little less voltage it sucks power in from AC input for charging batteries.

There is a three way connection node. AC input, AC output, and InverterAC. With current sensing the inverter can manage the AC current flow between the three branch node. This means inverter can totally power loads by itself, suppliment incoming AC input power for output AC loads (load shaving), or even supply loads and push power out backwards into grid (export to grid). It can also suck power from grid or generator to charge battery. AC coupling allows a PV grid tied inverter connected in parallel with hybrid inverter output to push power into AC out to either push power through to grid or through inverter to charge battery. For AC coupling the hybrid inverter acts as as surrogate grid for PV grid tied inverters when grid goes down.

Generator AC must be stable. A common problem with 3600 rpm engine generators is their freq stabilty wobbles because of unstable govenor. Inverter phase tracking is relatively slow so it cannot follow a fast varying AC input frequency/phase. Inverter will just open relay and release from generator when this happens.

Block diagram looks like this. This diagram happens to be for low freq inverter but HF inverter is similar for block diagram.

inverter functional diagram.png
 
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Bud Martin

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Wow! Do you have the link for me to get more info of the diagram? I would like to download and study them, thanks!
 

RCinFLA

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Its a drawing I did. Here it is in PDF.
 

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toms

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You are referring to a battery equipped hybrid inverter that accepts AC input from grid or generator.

Hybrid inverter has inverter permanently connected to AC output. There is a connect relay between AC input and inverter. Relay is normally open. When an AC input voltage is detected and within acceptable voltage/frequency range the inverter starts a slow phase tracking adjusment to match the AC input phase. The inverter also is adjusted to match AC input voltage which is the easy part. Once AC phase and voltage of inverter matches AC input the input relay closes putting the inverter in parallel with AC input (grid or generator).

PWM inverter can modify its AC voltage slightly when running in parallel wth grid. If inverter puts out a little more voltage then AC input voltage the inverter pushes out power. If inverter is adjusted to out a little less voltage it sucks power in from AC input for charging batteries.

There is a three way connection node. AC input, AC output, and InverterAC. With current sensing the inverter can manage the AC current flow between the three branch node. This means inverter can totally power loads by itself, suppliment incoming AC input power for output AC loads (load shaving), or even supply loads and push power out backwards into grid (export to grid). It can also suck power from grid or generator to charge battery. AC coupling allows a PV grid tied inverter connected in parallel with hybrid inverter output to push power into AC out to either push power through to grid or through inverter to charge battery. For AC coupling the hybrid inverter acts as as surrogate grid for PV grid tied inverters when grid goes down.

Generator AC must be stable. A common problem with 3600 rpm engine generators is their freq stabilty wobbles because of unstable govenor. Inverter phase tracking is relatively slow so it cannot follow a fast varying AC input frequency/phase. Inverter will just open relay and release from generator when this happens.

Block diagram looks like this. This diagram happens to be for low freq inverter but HF inverter is similar for block diagram.

View attachment 78665


Thankyou for that information. So the hybrid inverter slowly changes phase until it is synchronised with the generator?

How does this affect the loads attached to the inverter, are there any loads that will not like the slow phase change (eg electronics / motors)

Cheers
 

SpongeboB Sinewave

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The relay is not connected to the inverter supplying the AC output until the inverter is in phase and has matched its voltage to the input AC source.

So by the time the relay is engaged and the AC input source (generator or grid), and they are tied together, there will be no abrupt change to the AC output load.

The inverter doing the matching up of phase and voltage is aware of the AC input voltage properties on the input side of the relay contacts.

boB
 

toms

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The relay is not connected to the inverter supplying the AC output until the inverter is in phase and has matched its voltage to the input AC source.

So by the time the relay is engaged and the AC input source (generator or grid), and they are tied together, there will be no abrupt change to the AC output load.

The inverter doing the matching up of phase and voltage is aware of the AC input voltage properties on the input side of the relay contacts.

boB

I get that, but the two sources are both powering loads, and the phases don’t match. After synching, the phases match - and the whole time both sources were powering loads.

Somewhere along the line a load was being powered by a waveform that was changing phase. I’m wondering how this affects the load.
 

RCinFLA

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For a synchronous generator (a 3600 rpm, 60 Hz generator), if it has engine varying in rpm due to an unstable rpm govenor, its causes its AC output frequency to vary. This can be a problem for inverter to track. A sudden heavy load surge on generator requires the carborator throttle to be raised and there may be a momentary time delay in responseding to heavy load and therefore a dip in engine rpm. The inverter may release from generator under this condition. Inverter will reconnect to generator when its freq/phase, and AC input voltage stabilizes again.

It is important that hybrid inverter maximum AC input current limit user setting is set before connecting generator. If inverter's AC input current limit is set too high, based on wattage spec of generator, when inverter syncs and closes connect relay to generator, the inverter can jump on genertor with a load up to the max limit setup on the inverter. If this exceeds generator capability the generator will bog down, slump in rpm, causing inverter to immediately release from generator. This process will just keep repeating at a rate of every minute or two.

The inverter tracks slowly for a number of reasons. First, you really don't want AC phase to make abrupt changes for loads like AC motors as there will be current surges in AC motor. Second, the inverter PWM chopping by inverter to create a sinewave needs to be recomputed for voltage, frequency/phase shift and it must do this in small continuous increments without an abrupt jump in the increments. This requires a lot of recalculations, sensor checking, and adjusting MOSFET PWM chopping sequences. If generator wobbles more than about 0.3 Hz per second rate the inverter will not be able to track it. Inverter may refuse to connect to generator because it cannot achieve a continuous sync with generator or will release from generator if wobble happens after inverter has connected to generator. Input frequency range spec on inverter is not the same as acceptable AC input frequency wobble rate. Whatever frequency the AC input is, within acceptable range of inverter spec, it must also be relatively stable at that frequency.

Inverter detects voltage level and zero crossing to determine how to match AC input. It does not directly check waveform distortion of AC input source which may not be great for a cheap synchronous generator. Also, load on inverter AC out may have a load that has a power factor less than one. This means the output current is not necessarily in phase with output AC voltage so this is another complication on inverter sensor current measurements.

When zero crossing and voltage is tracked to the best ability of the inverter the final criteria for maintaining connection to AC input is peak current imposed on inverter. If the AC input wobbles in phase, drops or rises too much in voltage level, or has too much waveform distortion there will be current surge peaks in the inverter. When these current surge peaks get too great the inverter will open connect relay which releases tie between AC input and inverter. Inverter must then supply all AC output load.

Final tough problem for inverter is detecting when AC input goes away. When on grid for AC input the voltage is usually pulled down when grid goes down causing a momentary overload on inverter causing it to immediately release from grid AC input.

For a generator running out of gas there usually begins a sputtering surge of engine rpm that causes inverter to disconnect. There will be some momentary backfeed current into generator during this process so it is better if you avoid letting generator run out of gas, instead disconnecting generator electrical connection to inverter first, then stop generator and refuel generator.

Toughest thing for hybrid inverter to detect is an AC input breaker opening. Zero AC input current is allowed under certain conditions so inverter may not immediately open AC input connect relay. Usually the internal node current sensors in hybrid inverter feeding info to microcontroller will allow the microcontroller to figure out that there should be some current coming in or out of AC input based on AC out load and inverter power production, so it makes the decision to re-open input connect relay. This may not be immediately after opening AC input breaker however.

The inverter running in parallel with AC input gives a lot of options. These options are features within firmware of inverter and may not be supported by all hybrid inverters. For example, battery power through inverter can be summed with generator AC input power allowing more AC output loads than can be individually supported by generator or inverter alone. You can store PV power in batteries then push it to grid at a later time. For time-of-use grid tariffing, you can load shave power taken from grid, supplimenting AC house loads from battery power to avoid heavy grid power consumption during time of high grid power tariff.
 
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noenegdod

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I get that, but the two sources are both powering loads, and the phases don’t match. After synching, the phases match - and the whole time both sources were powering loads.

Somewhere along the line a load was being powered by a waveform that was changing phase. I’m wondering how this affects the load.
This depends on how fast you are trying to synchronize and why it can take up to 30 seconds or sometimes even longer to accomplish. If the shore power connection is rock solid (grid) then it is usually quite quick for the inverter to synchronize as it at maximum has only half the wave out (speed up or slow down) to match with. If the ac input is not consistent (some generators) it can take a long time.

It is much easier to synchronize a generator to the grid. You just spin you generator at 3601, wait till waves match, throw the switch and its done. pour the steam to the turbine and your making power. Simple as that (not really but close enough).
 

wattmatters

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This is an interesting thread.

My grid tied inverter (Fronius Symo), like most here in Australia, operates in parallel with the grid supply.

On start up it takes 1-2 minutes to begin supplying (3-phase) AC output, going through a process of grid synchronisation amongst other start up processes which it shows on the unit's display.

My off-grid PIP clone inverter also has a delay when cutting back to grid pass through mode, I guess about 30 seconds, while it sorts itself out.

Going the other way, cutting over from grid supply to battery/solar supply however is effectively instant, like a UPS. I have tested using it with my generator as my AC input and it works just fine, partly I expect because my generator (Yamaha EF3000iSE) is an inverter generator which probably has better control over output frequency.
 

SpongeboB Sinewave

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I get that, but the two sources are both powering loads, and the phases don’t match. After synching, the phases match - and the whole time both sources were powering loads.

Somewhere along the line a load was being powered by a waveform that was changing phase. I’m wondering how this affects the load.

This assumes that the generator is not powering anything and is getting ready to be a backup source.

But the generator could be powering something first as well. Just like the grid is powering the neighborhood. :) No different than connecting to the grid to charge the batteries or do grid support or grid tie.

If the two amplitudes or frequency are different, then the inverter's amplitude and/or frequency will change while it syncs to the AC source gen or grid. In that way the inverter's loads would change a bit
 
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toms

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Thanks everyone for your time in answering this question. I think i have a better idea now.
Just to make sure, is the following correct?

The hybrid inverter analyses the incoming generator signal to ensure it is stable and within acceptable voltage/frequency limits.

The hybrid inverter then slightly changes the frequency of its output until the waveforms of both sources match.

This small frequency change has no significant impact on any existing loads being powered by the hybrid generator,

If the generator waveform goes out of range it is automatically disconnected from the hybrid.

Thanks again everyone, especially @RCinFLA for your time and information.
 
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