# Paralleling Generator with Utility



## 2093lk

How does a system know to pull load from generator(s) when it is paralleled, will be supplementing with generators not feeding back to ulility


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## Big John

Voltage.

If your load is consuming more power than your generator can source, the voltage on your building bus begins to drop. Once it gets below the voltage available at your utility service point, you begin to consume power instead of shipping it.

The opposite is also true, that if your generator is large enough, it can produce more power than you can locally consume, which begins to push up the building bus voltage to the point that you slightly exceed utility voltage and being shipping power. 

The trick to both of these is that the utility connection point is what's called an "infinite bus": In the majority of cases, the power available from the grid is drastically greater than the power the generator can source (and for many purposes can assumed to be infinite) so the generator is not capable of significantly changing voltage levels beyond just enough difference to cause power to be supplied or consumed.


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## nolabama

^^ what he said. ^^^. Infinite buss about sums it up. That infinite buss is what syncs that generator too. I would like to work on a setup like that one day...


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## don_resqcapt19

I thought the generator tried to increase the frequency and not voltage to send power to the grid.


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## Mshea

don_resqcapt19 said:


> I thought the generator tried to increase the frequency and not voltage to send power to the grid.


 No screw with the frequency and the generator will go out of sync and disconnect.
Excitation grows the output current but the frequency is constant and must be in sync. the voltage rises and falls according to the control circuit which raises and lowers the magnetic field which raises and lowers the output voltage. The governor controls the frequency by controlling engine speed. As a generator overloads the engine or prime mover cannot keep frequency and the output gets disconnected if running in Sync with the utility.

So a stand alone generator can wander around 60 hertz without a lot of trouble from 56 to 64 hertz would only bug the clocks. Parallel to the utility must have excellent frequency control as would parallel to another generator. I recall the local hospital was never able to sync 1 of their generators so the hospital used this 1 generator for the heating plant standby power. I was building the new substation and this generator had to be rewound and parallelled to two other generators but the hospital was very concerned that their years of trying to sync this 1 unit never was successful. Turned out the three generators had 2 different governors and replacing the 1 oddball fixed it all. They could never be parallelled because the 1 governor had a different response time and could not stay in Sync.


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## don_resqcapt19

When you are in parallel with the utility you can't really change the frequency, but as you increase the input energy into your generator, it tries to increase the frequency, but since it can't it provides power to the grid.


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## don_resqcapt19

The following is from *this document*.


> As the fuel to the engine of one generator set in a group is increased it will not lead to an increase in speed and hence frequency (as it would if it were operating alone) but it will lead to an increase in the proportion of the total kW load that it will deliver.


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## John

nolabama said:


> ^^ what he said. ^^^. Infinite buss about sums it up. That infinite buss is what syncs that generator too. I would like to work on a setup like that one day...


Do you know how two locomotives are synchronized when they are connected together to form a train?


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## walkerj

John said:


> Do you know how two locomotives are synchronized when they are connected together to form a train?


Magic?


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## nolabama

I don't believe that they are synced. I think it works different than that. I will find out.


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## 3xdad

This voodoo you guys are discussing, is this how those PV systems work that are just backfeeding a breaker?


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## nolabama

3xdad said:


> This voodoo you guys are discussing, is this how those PV systems work that are just backfeeding a breaker?


Not really. The VooDoo is the engine and frequency of the gensets. PV systems do not have a rotating assembly to break if it goes downhill.

And it's my understanding the infinite buss forces the engine to the poco frequency. It does not have a choice in the matter. The fun supposedly starts when you sync two gensets. Inevitably you will have a lead/lag and that is when things can get interesting. 

My bridges have two motors that are basically the same. When the bridge spins one drive will show a hz difference of about 1 or 2 hertz. The programming is the same, but the motors are mechanically bound together. Apparently physics says one will lead and the same phenomena occurs when syncing gensets.


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## frenchelectrican

John said:


> Do you know how two locomotives are synchronized when they are connected together to form a train?


There are couple ways it can be done with synchronized diesel electric units but most used DC feedback loop on speed control circuit.

That how they uesally get them in synchronous but the newer diesel electric units are AC instead of DC but still use the DC feedback loop unless you have pair of AC units then it will be much easier.

But there one thing you will keep in your mind it will useally run in lead/lag mode most case so when the engineer hit the notch or run ( throttle ) lever the lead unit will respond first then a split second the lag units will catch up. ( as long the lead engine still run but once the lead engine drop out that is differnt game to deal then it will be dead stick mode which most engineers don't like it much ) 

Merci,
Marc


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## BBQ

3xdad said:


> This voodoo you guys are discussing, is this how those PV systems work that are just backfeeding a breaker?





nolabama said:


> Not really. The VooDoo is the engine and frequency of the gensets. PV systems do not have a rotating assembly to break if it goes downhill.


Actually other than no rotating parts I believe it is exactly the same.


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## nolabama

BBQ said:


> Actually other than no rotating parts I believe it is exactly the same.


Yeah you right but it's no fun. Their is no VooDoo involved cause nothing will break.


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## ScooterMcGavin

Synchronous generators run it what is called speed droop mode (constant speed) vs isochronous mode (constant frequency.) The control thats used to adjust the MW output of a synchronous generator is referred to as the governor speed changer. As long as the generator is connected to the grid it will be in sync with the grid. No ifs ands or buts about that. If the generator is brought on to the grid out of sync it will be forced (sometimes violently) into sync with the grid.

As mentioned previously on an infinite buss a single generator can't effect the frequency or voltage of the grid. When a generator comes up to speed and parrallels its does whats known as "floating the line" the prime mover is making just enough power to keep the generator from motoring but not really putting any power on the grid yet. As you run the governor up and start putting more torque on the generator it will start sending power out to the grid. 

The governeror is set for a no load speed. The speed the generator would go if it weren't synced up to the grid. If the generator were to suddenly become disconnected from the grid it's speed would increase quickly to its no load speed, and it could easily be destroyed. There are backups upon backups for this overspeed condition which has to stop the prime mover immdediatly. (Think two people pushing as hard they can against each other and then one suddenly lets go).

The opposite condition is when you lose your prime mover and the generator becomes a motor which is also known as reverse power. A generator makes a terrible motor and it will get hot and take a lot of VARS in. Believe it or not in certain cases the generator will be allowed to motor temporarily when a power plant trips because that way you know the power from the prime mover has been successfully shutdown and you can now safely disconnect from the grid without worrying about overspeeding.

As previously mentioned raising the excitiation of the generator will raise the voltage of the generator but it won't raise the voltage of the grid. If the generator voltage is greater than the grids than the generator is said to be overexcitied and will push VARS out on to the grid. Likewise if the generator voltage is less than that of the grid the generator will take in VARS from the grid and it basically acts as a capacitor. 

If the grid as a whole becomes overloaded the Generators will start to slow down and they will trip out on underfrequency which can have a cascading failure effect.

The AVR adjusts the excitation to maintain a constant generator voltage. The more load the generator picks up the more excitation you need on the generator to maintain a specific voltage.


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## Jerome208

John said:


> Do you know how two locomotives are synchronized when they are connected together to form a train?


Locomotives are totally independent electrically except for some control wiring between them.

They have 8 discrete notches of throttle position and when the lead unit is in #6 all the following units will be in #6 and each will be putting #6 power to its own motors without caring what the others are doing or not doing.

Some are old, some are new, some are AC, some are DC. It would be a nightmare to try to connect power between them.


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