Self excited induction generators and grid connected solar inverters?

[looking]

TL;DR Summary

No certified commercial grid connected wind inverters are readily available for constant voltage generation from conventional standby natural gas generators ( rectified to DC of course).....so maybe properly sized self excited induction motor generators would provide the constant current that grid connected solar inverters could handle.

OK I've developed a workable grid tied "wind" inverter, Certified, installed and successfully operating waste flare gas utility grid connected net metering system, with its associated functional usable space heating appreciated with todays -40 degree wind chills.
All this fully described within this forum In the past 5 years or so.

But surely increased generating capacity improvements can be made, such as using a single three phase inverter instead of three single phase inverters and transformers combined together, and maybe even using an a natural gas engine coupled to self excited common 3ph induction motor genset power end whose output would be compatible with ubiquitous solar inverters. Again mainly because 3 to 100 kw certified "wind" inverters just aren' even made anymore.
The three phase AC to DC rectification has already been fully tested and implemented with the existing working versions. Thus no new changes need to be made.

Now I know that asynchronous induction motors need a source of reactive power to even begin outputting the mechanical power of a coupled internal engine (or steam or wind or water or whatever) and the electrical grid would be almost an ideal source except for the most important matter of some once in a lifetime grid repairman safety issue.
So does anyone have a self excited stator connected capacitor solution that still produces a fairly voltage stable output. Currently I favor a stator connected wye connection capacitor network but don't know how many microfarads to try start testing. The initial test induction "motor" will be 20hp and the solar inverter 208v three phase to the grid.

Long story short. Spinning a 1740 rpm induction motor at a governed speed of about 1850 rpm will produce a constant current of about exactly the running amps of the induction motor if it were connected to a 3 phase grid.
I know the AC rectification to DC works.
Generator speed will be tightly controlled to within a few rpms.
If voltage and current output is stable, the the solar inverter should be being fed exactly what would keep it happy, I'd expect success, and extreme simplicity.

And I'll almost bet that an independent certifying agency like Intertec, and the electrical grid operator and associated inspectors will all be satisfied because the solar inverter addresses all their safety concerns of grid connection.
Or is there unanimous consent of electrical engineers that something serious is being overlooked. If so please speak up. Thanks
I'm certainly open to better ideas, and especially solutions that are orders of magnitude simpler, more efficient and less costly. Thanks.

 

[Baluncore]

First, clearly specify the problem.
You have an IC engine, fuelled by waste gas. It will deliver shaft torque to a rotating machine, and you want that rotating machine, directly or indirectly, to generate a stable direct current, at high DC voltage, that appears to a type-approved grid-tie inverter, like a string of PV panels.
Is that correct ?

 

[Averagesupernova]

I'm not completely clear on the task either. What caught my eye was spinning an induction motor at 1850 rpm while it is apparently hooked to the commercial power.

 

[Baluncore]

What caught my eye was spinning an induction motor at 1850 rpm while it is apparently hooked to the commercial power.

I interpreted the need for excitation capacitors, was because it was NOT grid connected, but a local energy supply, that could then be rectified for DC supply to the grid-tie inverter.

An induction generator, connected to the grid, does not need capacitive reactance to self-excite it, but I don't know the regulations regarding connection. Grid capacitance may be sufficient to self-excite them when there is a disconnect.

The RPM of a self-excited induction generator, is simply determined by resonance of the stator inductance, with the capacitive load. I think one question from above is; How is the output voltage of a self-excited induction generator regulated?

 

[looking]

First, clearly specify the problem.
You have an IC engine, fuelled by waste gas. It will deliver shaft torque to a rotating machine, and you want that rotating machine, directly or indirectly, to generate a stable direct current, at high DC voltage, that appears to a type-approved grid-tie inverter, like a string of PV panels.
Is that correct ?

Yes... exactly except for your words "at high DC voltage". The voltage produced by the "rotating machine" would be AC and would be a characteristic of an "excited" induction motor driven at speeds a few percent faster than normal electric motor rpms, which makes them into an electrical generator. Any help as to delta stator winding connection of suitable capacitors etc would be appreciated. Stable nominal voltage and constant current from a 10 to 20 ish HP indiction generator is the objective.


If that is a clear answer the please don't read any of the following repetition. I do appreciate your time and past helpful knowledge. I also just made an edit on the stator capacitance since the connections should probably be "delta" instead of "wye".
Murray

FURTHER CLARIFICATION IF REQUIRED
IC engine fueled by waste gas, delivering torque to 208 voltage AC induction motor. Object is for this setup to " self excite" and produce power output when the IC engine speed is increased to cause the driven induction motor to act as a generator producing AC power.

Note that this IS a characteristic of any off the shelf 3 phase induction motor. As others have stated, if this IC engine/ induction motor setup were to be connected to a 60 cycle grid, everything would work perfectly when any nominal 1740 rpm induction motor was driven to about 1840 rpm, and the project objectives would be achieved.

HOWEVER THE PROJECT WOULD UTTERLY FAIL BECAUSE OF THE POSSIBILITY THAT SOME COMBINATION OF STRAY CAPICITANCE , IN SOME RARE INSTANCE COULD RESULT IN A SAFETY HAZARDS.

One solution to address that problem is to use any grid certified solar inverter in the setup, in order to totally isolate the IC driven/ induction motor generator from the grid.

Please note that the words DC have not yet been mentioned anywhere yet. DC comes in only because the grid connected "solar" inverter needs to be fed DC constant current. Trust me in saying that I know how to do that with a simple 3 phase rectifier that is proven to work, and will be acceptable to Intertec, electrical inspectors and the utility grid that this IC engine/induction generator/rectifier/ grid certified "solar" inverter will involve.

The above explanation is for the benefit of others.
My inquiry is for suggestions as to how to implement he self excitation of the IC engine/driven motor to produce the reactive power needed since the solar inverter is being used to totally safely isolate the power plant from the utility grid.

 

[looking]

TL;DR Summary: No certified commercial grid connected wind inverters are readily available for constant voltage generation from conventional standby natural gas generators ( rectified to DC of course).....so maybe properly sized self excited induction motor generators would provide the constant current that grid connected solar inverters could handle.

OK I've developed a workable grid tied "wind" inverter, Certified, installed and successfully operating waste flare gas utility grid connected net metering system, with its associated functional usable space heating appreciated with todays -40 degree wind chills.
All this fully described within this forum In the past 5 years or so.

But surely increased generating capacity improvements can be made, such as using a single three phase inverter instead of three single phase inverters and transformers combined together, and maybe even using an a natural gas engine coupled to self excited common 3ph induction motor genset power end whose output would be compatible with ubiquitous solar inverters. Again mainly because 3 to 100 kw certified "wind" inverters just aren' t even made anymore.
The three phase AC to DC rectification has already been fully tested and implemented with the existing working versions. Thus no new changes need to be made.

Now I know that asynchronous induction motors need a source of reactive power to even begin outputting the mechanical power of a coupled internal engine (or steam or wind or water or whatever) and the electrical grid would be almost an ideal source except for the most important matter of some once in a lifetime grid repairman safety issue.
So does anyone have a self excited stator connected capacitor solution that still produces a fairly voltage stable output. Currently I favor a stator connected delta connection capacitor network but don't know how many microfarads to try start testing. The initial test induction "motor" will be 20hp and the solar inverter 208v three phase to the grid.

Long story short. Spinning a 1740 rpm induction motor at a governed speed of about 1850 rpm will produce a constant current of about exactly the running amps of the induction motor if it were connected to a 3 phase grid.
I know the AC rectification to DC works.
Generator speed will be tightly controlled to within a few rpms.
If voltage and current output is stable, the the solar inverter should be being fed exactly what would keep it happy, I'd expect success, and extreme simplicity.

And I'll almost bet that an independent certifying agency like Intertec, and the electrical grid operator and associated inspectors will all be satisfied because the solar inverter addresses all their safety concerns of grid connection.
Or is there unanimous consent of electrical engineers that something serious is being overlooked. If so please speak up. Thanks
I'm certainly open to better ideas, and especially solutions that are orders of magnitude simpler, more efficient and less costly. Thanks.

 

[looking]

I'm not completely clear on the task either. What caught my eye was spinning an induction motor at 1850 rpm while it is apparently hooked to the commercial power.

Connected Or hooked .... NO..but isolated by a grid certified inverter. ...yes
If only it could safely be directly connected, but that isn't an option AND ABSOLUTELY NO ONE SHOULD DO IT. It does make you wonder how a grid connected drag line or crane might potentially turn into an induction generator if the power source failed and the load turned a potential prime mover.

 

[looking]

I interpreted the need for excitation capacitors, was because it was NOT grid connected, but a local energy supply, that could then be rectified for DC supply to the grid-tie inverter.

An induction generator, connected to the grid, does not need capacitive reactance to self-excite it, but I don't know the regulations regarding connection. Grid capacitance may be sufficient to self-excite them when there is a disconnect.

The RPM of a self-excited induction generator, is simply determined by resonance of the stator inductance, with the capacitive load. I think one question from above is; How is the output voltage of a self-excited induction generator regulated?

Agreed, all the way down to your last paragraph.

But then what are the apparatus and formula is needed to create the self-excited state for a particular 20 hp induction motor driven at a constant speed by a prime mover; and if the induction generated voltage is rectified and intrinsically stable when feeding into the constant current sink of say a solar inverter; then a solution has been achieved

 

[Baluncore]

Yes... exactly except for your words "at high DC voltage". The voltage produced by the "rotating machine" would be AC and would be a characteristic of an "excited" induction motor driven at speeds a few percent faster than normal electric motor rpms, which makes them into an electrical generator.

You have constrained the generator to be an induction machine, indirectly generating DC by use of a rectifier. I allowed a dynamo or alternator to be considered, since the only requirement appears to be sufficient DC voltage to satisfy a grid-tie inverter.

Any help as to delta stator winding connection of suitable capacitors etc would be appreciated. Stable nominal voltage and constant current from a 10 to 20 ish HP indiction generator is the objective.

Follow the standard procedure to select the excitation capacitors.
https://en.wikipedia.org/wiki/Induction_generator#Example_application

In effect, you must overcompensate the IM inductance, to get a power factor greater than unity at the specified RPM and supply frequency. If you measure the IM current when free running, (easy), and when the rotor is physically locked, (be very quick), you can compute an estimate of the resistance and the inductance of the IM windings.

Be very careful.
High voltages can appear suddenly, when you least expect it.

 

[looking]

You have constrained the generator to be an induction machine, indirectly generating DC by use of a rectifier. I allowed a dynamo or alternator to be considered, since the only requirement appears to be sufficient DC voltage to satisfy a grid-tie inverter.


Follow the standard procedure to select the excitation capacitors.
https://en.wikipedia.org/wiki/Induction_generator#Example_application

In effect, you must overcompensate the IM inductance, to get a power factor greater than unity at the specified RPM and supply frequency. If you measure the IM current when free running, (easy), and when the rotor is physically locked, (be very quick), you can compute an estimate of the resistance and the inductance of the IM windings.

Be very careful.
High voltages can appear suddenly, when you least expect it.

Excellent. The only reason I would prefer an induction generator is that there are 30 to 100 horse power "induction motors" available at even no cost.
THANKS for the link.