MICRO GRID

[loren1115]

We have a 24KW ST head direct coupled to a DET 4-53 runing at 480volts 1800 rpm,We are thinking of putting in a RV park,we have a mothballed DETROIT V12-71 505HP,if we drove a very large 480v 3ph motor with it would it synch with with the 24k head and be happy with each other? We're 65 miles from the grid and we're getting numbers like $30,000 a mile to be conneted.Welcome to West texas.

thanks Loren

[Victor]

In principal yes. Here are some warts IMO: 1. The induction generator needs to be run at above the speed of the "grid" to produce power. In all the systems that I'm familiar with the torque on the prime mover of the induction generator is less than the torque of the generator and is the limiting factor, if it is exeeded the generator becomes unstable and over speeds. 2. If the induction generator is large relative to the synchronous generator or the load is  too small for the induction generator's prime mover the sychronous generator will try to overspeed, it's prime mover will be drug along,(I have heard tales of raw fuel in the exaust)

 My direct experience with induction generators is limited to a 25' diameter wind turbine generating into the real grid. I have heard of problem 2 in off grid wind diesel hybrid systems. Usually they have a selfadjusting dump load (heaters) to insure  system stability in high wind, light load conditions.

 

[iFred]

Any AC motor run by about 10% or so above the rated speed of that motor will begin producing AC. in other words.. if you ran the motor at around 1900 it should produce full 480 volts. This is what most people are doing with the listers or diesel engines and the DC motor to AC motor combinations. Most of the problem centers around Capacitors that must be rated and sized properly. Also this produces an output that is perfectly Sinewave, no inverter or batterys required.

[wpowokal]

In my opinion there is only one way to do it, buy a generator to suit the motor with syncronysing equipment, do it once and do it right.

If clients equipment started burning out and they perceived it may have been due to the power supply at that last park, well.........

Probally dosen't help much but just my thoughts.

regards Allan

[Flux]

I tend to agree with Victor. If the inductiom machine is large compared with the synchronus one it will be unstable.

Flux

[LEXX]

I have an idea but it might be totally wrong and it hinges on one question; why are you using 480 volt?  do you have a system for your house based on this?  If so, do you want to tie the whole thing together?  maybe you should give us a detailed description of your entire system; inverters, batteries?, transformers, whatever you have then we can actually give you some advice that will make some sence.  Also tell us what you think the after-mod output needs to be.  Love to help but I need some more info.

LEXX

[bob g]

i think trying to keep that system stable is going to be a challenge,

mainly because of the larger unit needing to synch to the smaller one.

i think the reason that induction generation works so well on a grid connected system is the massive difference between the grid and the tremendously smaller induction generator system,

the relatively smaller induction generator when connected to the grid is going to find it impossible to overspeed and go over the grid frequency, or to raise the line voltage much over what it is already, except for possible increases in line voltage at the point of hookup.

your primary system with the 53 series detroit cannot contain or maintain line parameters when faced with the 12v71 detroit driving a much larger induction generator.

i think your frequency controll is going to be poor, and trying to keep things in control in general are going to be a nightmare without some rather sophisticated managment systems.

bob g

[srnoth]

http://www.qsl.net/ns8o/Induction_Generator.html

[srnoth]

Tried to post this yesterday, didn't work, so here I go again:

Hey guys,

The idea of using a large induction motor as a generator does has been tried and does work. Check out this site: http://www.qsl.net/ns8o/Induction_Generator.html

Basically, you don't even need the 24KW head to keep the induction generator at 60Hz. What you do, if I understand correctly, is that you use a throttle governor to control the speed of the engine and generator, which is what controls the frequency, so you set the governor to maintain the speed that gives 60Hz. Then, to control the voltage, you add capacitors until the no-load voltage is about 125V, and you can then load the generator until the voltage reaches about 105v, at which point you are at maximum load. The site I gave you has a lot more information.

Anyway, using the above method, the induction generator would have to be on a different circuit from the 24KW head you already own. I suppose it is possible to run the induction motor off the 24KW head, and then run the induction motor at slightly above the rated speed, but this could cause problems. The reason that using an induction generator with the grid works is because the grid is so large, that one extra generation source is not going to affect the load on the others much. The problem with the 'micro grid' you are designing is that you only have two generation sources, which means that it is quite likely that when you speed up the induction motor so it starts generating, it may very well turn the 24KW head into a motor, and both motors could over speed.

However, I do see a way in which this could work. If you have the 24KW head running all the time, then you can build a load sensing circuit do as follows:

1. When the load is very small (<24KW), the 24KW generator would be all that was running.
   
2. When the load neared 24KW, the voltage would start to drop, and the load sensing circuit would pick up this voltage drop, and start the large induction generator.
   
3. The load sensing circuit would then control the throttle of the induction generator so that if it only needed to take a small load off the 24KW generator, it would only run slightly above the rated speed. As the load continues to increase even more, the throttle would increase to keep up with the load.
   
   

Basically what this circuit would do is attempt to keep the voltage at 120V, so therefore it would only increase the throttle speed, and thus the output amperage enough to keep up with the load, and never enough to send current back into the 24KW head.

If you can find a way of doing this, it should work.

Cheers,

Stephen.

[srnoth]

And a thought about the two generators getting out-of-sync, this would be impossible because it would put too much load on the motor, causing it to slow down and thus remain in sync.